TABLE OF CONTENTS SECTION

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Introduction

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Conceptual Model of Recharge Potential – Chicot Aquifer

1

Hydrostratigraphy of the Chicot Aquifer

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Age-Dating Study by the U.S. Geological Survey

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Basis for Age-Dating with CFCs

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Estimated CFC Ages

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Significance of CFC Age Estimates

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Ground-Water Age-Dating Using Tritium and Radiocarbon

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Tritium

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Radiocarbon

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The Role of Deuterium and Oxygen-18 in Hydrologic Investigations

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Wells Sampled for Lafayette Parish Age-Dating Study

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Distribution of Wells

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Collection of Samples

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Results of Isotopic Analyses

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Tritium

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Radiocarbon

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Deuterium and Oxygen-18

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Discussion of Isotope Data

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Tritium and Radiocarbon

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Deuterium and Oxygen-18

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Final Draft Age-Dating of Lafayette Parish Ground Water: a Basis for Understanding Recharge and the Potential for Contamination Introduction The program of work described in this report was designed to add to the understanding of recharge potential, the age of ground water, and the potential for contamination of ground water in different sands of the Chicot aquifer in Lafayette Parish. Supporting data were collected specifically for this investigation and from studies conducted by the U.S. Geological Survey (USGS) (Fendick and Tollett, 2004; Tollett and Fendick, 2004). The program of work involved the analysis of samples of ground water for the naturally occurring radioactive isotopes of hydrogen (tritium, or 3H) and carbon (carbon-14, radiocarbon, or 14C) – both of which are widely used to ascertain whether ground water can be traced to recharge occurring within a recent or an older timeframe. Samples of ground water were also analyzed to determine the abundances of the stable isotopes oxygen-18 (18O), deuterium (2H), and carbon-13 (13C). Samples of water from a pond in easternmost Lafayette Parish were also analyzed for tritium, oxygen18, and deuterium. With regard to the specific interests of the LUS, the age-dating program establishes a sound basis for: 1. classifying the recharge potential of selected areas in the vicinity of wellfields owned by LUS and other areas of Lafayette Parish, 2. estimating the probable timeframe over which recharge occurs in the area encompassed by Lafayette Parish, and 3. evaluating the potential for the degradation of ground water by municipal, agricultural, and industrial sources of contamination. All ground water samples were also analyzed for the major dissolved solids which form the principal basis for characterizing the “quality” of drinking water. A detailed evaluation of the results of the major ion chemistry is found in a separate report. Conceptual Model of Recharge Potential – Chicot Aquifer The Chicot aquifer lies beneath 15 parishes of southwestern Louisiana (Figure 1), and it is the sole source of drinking water for customers of the LUS (Fenstermaker and others, 2002). Population growth and competition for ground water from the municipal, industrial, and agricultural sectors of the region’s economy have motivated many residents of southwestern Louisiana to express concern about contamination of drinking water and the ability of recharge processes to keep up with pumpage, especially during lengthy periods of drought. The concern over recharge potential, pumpage, and sustainability of the Chicot and other aquifers of Louisiana was a principal factor behind the passage of legislation in 2001, which was the basis for the development of a framework for a statewide groundwater management plan (C.H. Fenstermaker and others, 2002).

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Final Draft The Chicot aquifer extends westward into Texas. In Louisiana, the area of the Chicot aquifer with high to moderate recharge potential is located primarily in Rapides, Vernon, Beauregard, and Allen parishes (Figure 2). The medium to coarse-grained sands exposed at the surface in this area of the state provide pathways for the percolation of water to unconfined sands of the aquifer. The recharge potential in parishes to the south and east of the high-to-moderate recharge areas is rated as low. The uppermost part of the Chicot aquifer in these parishes consists of a cap of interbedded clay, silt, and fine-grained to very-fine grained sand. Idealized cross sections through southwestern Louisiana (Figures 3 and 4) show the low-permeability cap overlying the Chicot’s uppermost beds of sand and gravel. The cross sections also show the cap becoming thicker toward the south (Figure 3) and thinner toward the east (Figure 4). Most of the precipitation which falls on the surface of parishes with low recharge potential is diverted to streams which discharge water to coastal marshes. The comparatively small amount of water infiltrating the cap eventually reaches the underlying beds of sand and gravel which make up the uppermost water-bearing zone of the aquifer. The amount of recharge attributable to leakeage is highly variable, depending largely on the thickness of the cap and the presence of nearsurface sand bodies which provide pathways for the percolation of water to deeper sands. The aquifer is also recharged by the lateral inflow of water from the Atchafalaya aquifer, which is located to the east of the Chicot (Figure 4). The author of a Master’s thesis on the stratigraphy of the Chicot aquifer in Lafayette Parish (Williams, 1996) hypothesized that resistivity profiles of shallow wells indicate where the potential for recharge within the parish is greatest. The author identified an area in north Lafayette Parish (the François Coulee drainage network near the City of Carencro) where the resistivities of the upper sands of the Chicot are greater than resistivity profiles in other areas of the parish (Figure 5). Although a compelling basis for delineating the drainage network as a probable recharge area of the Chicot aquifer in Lafayette Parish, the study by Williams (1996) does not address the question of the time required for surface water to replenish the aquifer within the drainage network. Thus, while geophysical logs offer indirect evidence of greater recharge potential in north Lafayette Parish, there is no sound basis for inferring from the profiles that recharge within the drainage network occurs over a period of one year, 20 years, 40 years or longer. Hydrostratigraphy of the Chicot Aquifer The Chicot aquifer is a regionally extensive system of sand and gravel interbedded with confining to semi-confining beds of clay (Jones and others, 1954). Between the northernmost extent of the aquifer and the coastal parishes of Louisiana, the sediments which make up the system dip and thicken toward the south (Figure 3). This pattern is traceable to the depositional systems of the ancestral Mississippi and other rivers which laid down thick sections of sediment in stacked deltaic and interdeltaic sedimentary sequences. Along the North-South line of section (Figure 3), the Chicot increases in thickess from less than 100 ft in southern Rapides Parish to more than 1,500 ft in Vermilion Parish. Along the West-East cross section (Figure 4), the amount of fine-grained material increases toward the west. Between Lafayette Parish and Jefferson Davis Parish, the aquifer system is divided into two

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Final Draft sections referred to as the “Upper” and “Lower” sands. The division between these sands is usually based on beds of clay which form a confining to semi-confining layer within the Chicot (Jones and others, 1954). The clay is of variable thickness and is not present at all locations, so that the distinction between the Upper and Lower sands is not clearly apparent in all of the parishes which overlie the eastern reaches of the aquifer. In Calcasieu Parish and southward to Cameron Parish, the aquifer is divided into three major sand packages which are referred to as the “200-ft”, “500-ft”, and “700-ft” sands. The cross sections (Figures 3 and 4) illustrate not only the structure and stratigraphy of the formations which make up the Chicot aquifer, but also the lateral and vertical boundaries between fresh and saline waters of the aquifer system. The fresh water sands in each cross section are colored blue, and the sections with saline water are colored red. The transition zone between the two becomes shallower toward the south, such that the depth to saline water in parts of Vermilion Parish is 500 ft or less below sea level (Figure 3). The distinction between fresh water and saline water is typically based on the drinking water standard for chloride (250 milligrams per liter (mg/l)) and/or total dissolved solids (500 mg/l). Along the West-East cross section (Figure 4), saline water is found in the lowermost sands of the Chicot and the lowermost beds of the 700-ft sand. The figure depicts sections of the aquifer in which saline water might flow upward into the fresh water beds of the Lower sand. Upwelling of saline water might occur as a result of the difference in head between the fresh water and saline water sands and/or pressure sinks caused by pumping of wells which produce water from the Lower sand. Before the aquifer was extensively developed to support demand from the agricultural, industrial, and municipal sectors of the economy of southwestern Louisiana, the direction of ground water flow in the Chicot was southward (Jones and others, 1954); but pumpage from the Upper sand by farmers in Evangeline, Acadia, and Jefferson Davis parishes created a cone of depression which reversed the southward regional gradient (Figure 6). Chicot ground water now flows toward the areas of greatest pumpage, instead of toward the natural discharge areas of the coastal parishes. In Lafayette Parish, the direction of flow is toward the west. The westward gradient beneath Lafayette Parish has induced lateral inflow of ground water from the Atchafalaya aquifer (Jones and others, 1954). The location of the Atchafalaya aquifer with respect to the Chicot is illustrated by Figure 4. The Atchafalaya aquifer is hydraulically integrated with the Upper sand of the Chicot in easternmost Lafayette Parish. The approximate boundary between the two aquifers is the Coteau Ridge, a north-tosouth escarpment which forms a prominent topographic divide in Lafayette Parish. The ridge is the western terrace of an ancestral Mississippi River flood plain. Elevations are higher west of the ridge. Throughout its sinuous pathway in eastern Lafayette Parish, the ridge rises from 15 to 40 ft above the lower-lying areas to the east (Murphy and others, 1977). The ridge is prominently expressed along the 1000 block of Hector Connolly Road (between LF-CC-3 and LF-BD-1; Figure 7). Age-Dating Study by the United States Geological Survey In a study of the quality of water from 28 wells in Lafayette Parish and 27 wells from other parishes overyling the Chicot aquifer, the United States Geological Survey (USGS; Fendick and Tollett, 2004; Tollett and Fendick, 2004) analyzed samples of water from 29 wells for the concentrations of chlorfluorocarbons (CFCs) (Figure 8). The wells in Lafayette Parish ranged in depth from 11 ft to 79 ft below ground surface (bgs), and water depths were from 2.50 to 61.55 ft bgs. The wells in the

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Final Draft investigation of other Chicot aquifer parishes ranged in depth from 12.25 to 92 ft bgs, and the associated depths to water were 0.57 to 80.19 ft bgs. The CFC analyses were the basis for the estimates of the range of ages of shallow Chicot aquifer ground waters. Nearly all of the wells in the above studies were screened over sands of the confined section of the aquifer. Basis for Age-Dating with CFCs Known by the more common name “Freon,” CFCs are stable, synthetic organic compounds which were developed as alternatives to the refrigerants ammonia (NH3), methyl chloride (CH3Cl) and sulfur dioxide (SO2). The first CFC (CFC-12, dichlorodifluoromethane) was produced in 1928. The second (CFC-11, trichlorofluoromethane) was produced in 1932, followed by others – primarily CFC-113 (trichlorofulorotrifluoromethane) (Elkins, 1999). The widespread use of refrigerants released large volumes of CFCs to the atmosphere. CFCs are dissolved by atmospheric moisture and then deposited on the surface with precipitation. CFCs are considered to be reliable age-daters of ground water because (1) atmospheric concentrations of the compounds over the past 50 years have been calculated, (2) the solubilities of the compounds are well known, and (3) the concentrations of CFCs in water and in air can be measured with precision by gas chromatography. After the concentrations of CFCs in water are measured, an equivalent atmospheric concentration for each CFC is calculated as a gas-specific, temperature-dependent solubility function. The equivalent atmospheric concentrations are compared with a plot of atmospheric CFC concentrations versus time to determine the recharge date of a sample. The CFC age is then calculated by subtracting the year of recharge from the sample-collection date. Estimated CFC Ages The apparent CFC ages from the Lafayette Parish investigation are 12 to 50 years, with a median of approximately 32 years (Fendick and Tollett, 2004), and the apparent ages from other Chicot aquifer parishes are 17 to 49 years, with a median of approximately 38 years (Tollett and Fendick, 2004). Significance of CFC Age Estimates The apparent CFC ages reported by Fendick and Tollett (2004) and Tollett and Fendick (2004) indicate that meteoric waters which fall on or flow across the surface of the confined sections of the Chicot do not percolate rapidly through the confining clay to recharge the uppermost sands of the aquifer. More specifically, the age estimates suggest that complete inflitration of the cap is probably a minimum of three or more decades, depending on the thickness of the confining layer and the ratio of clay to silt and sand in the stratigraphic section. The CFC data, however, do not provide insight into the ages of deeper ground waters of the Chicot aquifer. This study complements and expands upon the results of the investigations by Fendick and Tollett (2004) and Tollett and Fendick (2004) through its focus on wells with a wider range of depths and the use of radionuclides with different half-lives.

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Final Draft

Ground-Water Age-Dating Using Tritium and Radiocarbon Tritium and radiocarbon are used to trace the flow of ground water and to determine the time elapsed since recharge attributable to precipitation. Both tracers enter ground water systems through natural recharge pathways. Tritium is an effective tracer of ground waters with recharge signatures of 60 years or less, and radiocarbon is used to identify ground waters with apparent recharge ages ranging from recent to between 20,000 and 40,000 years. Analyzing for both isotopes establishes a sound basis for determining whether recharge ages are dominantely recent, old, or a combination of recent and old. Tritium A naturally occurring isotope of hydrogen (H), tritium is produced in the upper atmosphere by the interaction of nitrogen-14 (14N, a stable isotope of the element nitrogen) with neutrons (n) generated by cosmic radiation: 14

N + n → 12C + 3H

eq. 1

The collision causes nitrogen-14 to split into an atom of carbon-12 (12C, a stable isotope of the element carbon) and tritium. Tritium is also produced by nuclear reactors and by the detonations of thermonuclear bombs. Because it is an isotope of hydrogen, tritium is incorporated as a substitute for one or both of the hydrogen atoms in water: 3HHO or 3H2O. The concentration is measured in Tritium Units (TU), where one TU is equal to one atom of tritium in 1018 atoms of hydrogen. The average natural rate of production of tritium in the northern hemisphere is estimated to be approximately 5 TU per year (Mazor, 1997). With a half-life of 12.32 years, tritium is regarded as a reliable indicator of recharge ages up to 60 years. Effects of Above-Ground Nuclear Weapons Tests Above-ground nuclear weapons tests conducted between 1951 and 1963 increased the production of tritium substantially above natural rates in the northern hemisphere. After 1963, the deposition rate of tritium decreased to nearly pre-bomb levels, as indicated by a long-term monitoring program conducted by the International Atomic Energy Agency (Global Network of Isotopes in Precipitation – http://isohis.iaea.org). Figure 9 traces the concentration of tritium in precipitation at Ottawa, Canada, on a monthly basis over a 47-year period (1953 – 2000). The monthly time-series graph shows the concentration of tritium increasing from approximately 10 TU in 1953 to more than 5,000 TU in 1963/64, and decreasing thereafter to levels consistent with pre-bomb activity by the middle to late 1990s. Closer to Lafayette Parish, tritium in precipitation at Waco, Texas, and Baton Rouge, Louisiana, peaked during the 1963/64 period (Figure 10). Measurements at Waco were made over 25 years (1961 – 1986). The measurements from samples collected at Waco increased to nearly 2,000 TU by 1963 and decreased to less than 10 TU by 1986. Measurements at Baton Rouge were made over an 18month period between 1963 and 1964. The maximum concentrations at Baton Rouge reached nearly 1,700 TU in 1963 and decreased to approximately 120 TU by the end of 1964.

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Final Draft

As a screening guideline, tritium concentrations as low as 0.1 TU are interpreted to indicate recharge dates extending back to the middle to late 1940s. Concentrations greater than 10 TU are considered to indicate the presence of bomb-generated tritium, and, hence, the occurrence of recharge during or shortly after the cessation of above-ground nuclear weapons tests. Concentrations between 1 and 10 TU are indicative either of recent recharge or of the mixing of old (depleted) and younger (tritiated) ground waters. Radiocarbon Radiocarbon, a radioactive isotope of the element carbon (C), is formed in the upper atmosphere by the interaction of nitrogen-14 (14N) with the flux of neutrons generated by cosmic radiation. The collision of neutrons with nitrogen-14 produces radiocarbon (14C) and hydrogen (H): 14

N + n → 14C + H

eq. 2

Carbon mixes with oxygen in the atmosphere and in the shallow soil zone to form carbon dioxide (CO2), which reacts with water in the soil zone to form carbonic acid (H2CO3): H2O + CO2 → H2CO3

eq. 3

As a weak acid, carbonic acid dissociates (loses one hydrogen atom) to form the dissolved ionic species bicarbonate (HCO3-): H2CO3 → H+ + HCO3-

eq. 4

It is the carbon in HCO3- which is isolated for analysis of radiocarbon by accelerator mass spectrometry (AMS). Because of its 5,730-year half-life, radiocarbon is used to date ground waters with apparent ages ranging from recent to as much as 40,000 years (Mazor, 1997). AMS yields precise measurements of the mass of radiocarbon in a sample of ground water. The activity (in decays per minute per gram of radiocarbon) of the sample is compared with the activity of radiocarbon in oxalic acid to yield a ratio known as Percent Modern Carbon (PMC), the standard reporting unit. The decay of radiocarbon is described by the radioactive decay equation: A = Aoe-t such that, A = the measured PMC in a sample of ground water, Ao = the initial activity of radiocarbon in the sample, assumed to be 100 PMC, e = natural log (base e),  = decay constant (ln(2)/5,730, where 5,730 = half-life in yrs of radiocarbon) t = time since beginning of radioactive decay

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eq. 5

Final Draft

From the radioactive decay equation, it can be shown that the amount of radiocarbon remaining in a sample of ground water decreases by 50 percent every 5,730 years. Correcting Radiocarbon Ages Age estimates derived from the radioactive decay equation (eq. 5) represent maximum apparent ages, based on the assumption that radiocarbon in the sample changes (decreases) only as a function of decay, and not as a result of mixing with other radiocarbon-depleted waters or by reactions with mineralogic sources of dead carbon in the subsurface. Old carbonate rocks can be assumed to have no active carbon – that is, radiocarbon values are 0 PMC. Water and carbon dioxide (with a radiocarbon value of 100 PMC) which equilibrates with limestone (0 PMC) will yield bicarbonate with a theoretical radiocarbon value of 50 PMC: H2O + CO2 (100 PMC) + CaCO3 (0 PMC) -> Ca2+ + 2HCO3- (50 PMC)

eq. 6

Interactions with silicate rocks and other noncarbon-bearing rocks do not cause dilution of radiocarbon (Mazor, 1997). The dissolution of silicates and other noncarbon-bearing rocks, however, may lead to the sequestration of radiocarbon through the precipitation of secondary carbonate minerals. With reference to the reaction described by equation 6, if 50 PMC is assumed to be the actual radiocarbon value of a sample, the apparent age will be 5,730 years when, in fact, the sample is of recent age. The two most difficult problems stemming from the use of radiocarbon in ground water age-dating investigations are (1) to ascertain whether there is evidence of dilution, and (2) to determine whether and how to correct reported radiocarbon values. Many approaches have been proposed to account for the effects of the dilution of radiocarbon: e.g., Pearson (1965), Tamers (1967), Wigley (1975), and Fontes and Garnier (1979). The adjustment methods developed by the above researchers vary principally with with respect to how dilution is modeled (e.g., chemical or isotopic mixing, and/or isotopic mixing with various forms of isotope exchange with carbonate rocks or sources of organic carbon). Many of the methods require, as key input factors, data which are not readily available or easily obtained during the course of a field investigation. The uncertainty associated with corrected age estimates may range from a few percent to 20 percent or more (Muller and Mayo, 1986). Other approaches are based on empirical methods which yield estimates of Ao which are lower than 100 PMC (Vogel, 1970). This report employs an empirical method of adjusting Ao similar to that used by Vogel (1970). Described in detail in a later section of this report, this approach involves the integration of radiocarbon with tritium measurements to identify demonstrably modern waters, and analyses of the abundance of the stable isotope carbon-13 (13C) in ground water to ascertain whether reactions have occurred in the subsurface with sources of carbon which have altered original radiocarbon signatures. This approach is used to estimate a value of Ao which is less than 100 PMC.

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Final Draft The Role of Deuterium and Oxygen-18 in Hydrologic Investigations The stable isotopes deuterium (2H) and oxygen-18 (18O) are used in hydrologic investigations because they are effective indicators of the origin of water (e.g., meteoric, deep basin brine) and of the effects of processes such as evaporation and thermal exchange reactions with rocks (Drever, 1988; Faure, 1991; Mazor, 1997). Their abundances relative to the stable isotopes hydrogen (H) and oxygen-16 (16O) are reported as per mille (‰) deviations (2H and 18O) with respect to Standard Mean Ocean Water (SMOW), which has a value of 0 ‰. A number of environmental factors affect the abundance of each isotope relative to SMOW (Mazor, 1997). As moisture-laden air moves inland, precipitation becomes increasingly depleted in both deuterium and oxygen-18. This is referred to as the “continentality” effect. Precipitation also becomes depleted in both isotopes as air masses move over higher elevations (elevation effect) or in response to heavy precipition (amount effect). In each case, 2H and 18O become increasingly negative with respect to SMOW. Evaporation enriches water in both deuterium and oxygen-18 (evporation effect). During evaporation, the lighter fractions (oxygen-16 and oxygen-17) are concentrated in the vapor phase, which is removed by air currents. The assocation between deuterium and oxygen-18 in precipitation is described by the Global Meteoric Water Line (GMWL; Craig, 1961). Craig (1961) observed that 2H and 18O of precipitation from locations around the world tends to fall along a linear trend (Figure 11) described by the following equation: 2H = 818O + 10

eq. 7

The GMWL is the standard against which 2H and 18O values (-values) of water are compared. It establishes a high probability that waters with -values which plot along the GMWL are of meteoric origin. Deviations (especially linear deviations from the GMWL) are related to environmental and/or geological factors which enrich water in one or both isotopes (Mazor, 1997). Considering the above association between deuterium and oxygen-18 in precipitation, it has been hypothesized that waters which charged aquifers in the distant past and under different (colder or warmer) climatic regimes bear isotopic signatures attributable to fractionation under conditions which produced fractionation patterns different from those of the average Holocene (~ 10,000 years before present) climate (Dutton, 1989; Darling, 1997). Data from investigations of the ground water systems of the Southern High Plains (Dutton and Simpkins, 1989) and of the bolson aquifers of the TransPecos region of Texas (Darling, 1997), for example, have illustrated that the -values of demonstrably older ground waters of those regions are more negative than the -values of modern precipitation and of ground waters with modern carbon-14 and tritium signatures. The lower -values are hypothesized to have been related to precipitation in colder and wetter climates. Wells Sampled for Lafayette Parish Age-Dating Study Thirteen wells and one pond were sampled as part of this investigation (Figure 12). The designations of the wells and the pond are listed in column 1 of Table 1. The identifier indicates, first, the parish (LF = Lafayette), the owner of the well (LUS = Lafayette Utilities System), and then the well number 8

Final Draft assigned either by the owner or by LBG-Guyton. Public supply wells are owned by the North Water District (ND), Lafayette Utilities System (LUS), the City of Carencro (CC), and the town of Youngsville (YV). Private wells are owned by Mr. Tom Dailey (TD), the Derouen Landfill (DL), and Mr. David Bernard (DB). Mr. Bernard is also the owner of the pond (DBP). Distribution of Wells Three of the wells produce water from the Lower sand of the Chicot aquifer. Two of these wells (LFLUS-3 and LF-LUS-12) are major supply wells for the LUS. The third Lower sand well (LF-CC-4) is owned by the City of Carencro. All other wells but two are screened within the Upper sand of the Chicot. The two wells which do not produce water from the Chicot are LF-DB-1 and LF-TD-1. Located east of the Coteau Ridge, these well produce water from the Quaternary Alluvium overlying the Atchafalaya aquifer. Six of the Upper sand wells in this investigation supply drinking water to the City of Lafayette (LFLUS-1, LF-LUS-4), the City of Carencro (LF-CC-3, LF-CC-4), the North District Water Supply Corporation (LF-ND-1), and the Town of Youngsville (LF-YV-3). A seventh well (LF-VC-1) supplies nonpotable water to the Lafayette Visitors Center. All other Upper sand wells are privately owned (LF-DB-1, LF-TD-1, and LF-DL-1). The wells were selected, based on the need to have isotopic and chemical data over as wide an area as possible in Lafayette Parish. Collection of Samples Each well was purged at its maximum discharge rate for a minimum of 15 minutes before samples were collected. Two 500-milliliter (mL) samples were collected from each well for analysis of major metals (sodium, potassium, calcium, magnesium, and iron) and nonmetals (bicarbonate, sulfate, and chloride). At the time of collection, each 500-mL metals sample was treated with a solution of nitric acid to lower the pH to 2.00 or less. The samples were placed on ice and delivered to Southern Petroleum Laboratories (SPL) of Scott, Louisiana. SPL analyzed the samples and reported all concentrations in units of milligrams per liter (mg/L). After the wells were purged and before any samples were collected, pH, temperature, and conductivity were recorded with a field-calibrated Hanna Instruments (model 98130) combination pH/electrical conductivity meter. One 1-liter sample was collected from each well for analysis of tritium. The sample bottles were sent (overnight delivery by Federal Express) to the University of Miami Tritium Laboratory, Rosenstiel School of Marine and Atmospheric Sciences, Coral Gables, Florida. At the Tritium Laboratory, the samples were electrolytically enriched, and tritium activites were measured by low-level counting. The results were reported in Tritium Units (TU). Samples for analysis of radiocarbon and carbon-13 were collected in 1-L polyethyelene bottles and treated with a solution of sodium hydroxide (NaOH) to raise the pH to 11.00 or more. The treatment with NaOH fixed the dissolved inorganic carbon for chemical precipitation by Beta Analytic Laboratory of Miami, Florida. Radiocarbon and stable carbon analyses were by means of accelerator mass spectrometry (AMS). Radiocarbon was reported as Percent Modern Carbon (PMC), and carbon13 was reported as 13C based on the Pee Dee Belmnite (PDB) standard.

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Final Draft One 100-mL sample from each well was collected for Stable Isotope Ratio Analysis (SIRA) of the abundances of 2H and 18O. Coastal Sciences Laboratory of Austin, Texas, conducted the analyses. The abundances of deuterium and oxygen-18 were reported as per mille (‰) deviations (2H and 18O) with respect to Standard Mean Ocean Water (SMOW). Results of Isotopic Analyses Tritium Tritium (Figure 13; Table 2) ranges from a high of 3.45 TU (LF-CC-3) to not-detected (or 0 TU, LFLUS-4). Seven of the 13 samples (LF-ND-1, LV-VC-1, LF-CC-3, LF-CC-5, LF-YV-3, LF-DB-1, and LF-TD-1) have tritium values which are clearly consistent with bomb/post bomb (1951 to present) recharge dates. Three (LF-ND-1, LF-CC-3, and LF-CC-5) of the seven with bomb/post-bomb signatures are located in northern Lafayette Parish, in or around the François Coulee drainage area. Two (LF-DB-1 and LF-TD-1) are east of the Coteau Ridge. The others are located near the northernmost reaches of the City of Lafayette (LF-VC-1) and in the Town of Youngsville (LF-YV-3). The tritium signatures of the other six samples are more consistent with recharge which predates the bomb/post-bomb era. The greatest tritium values are associated with the shallowest of the 13 wells in the investigation (Figure 14). The depths of most of these wells are typically 150 ft or less. The wells with the lowest tritium concentrations are typically greater than 175 ft in depth. Well LF-DB-1 (Figure 12) is adjacent to a pond in eastern Lafayette Parish from which sample LF-DBP-1 was collected (Figure 15). The depth of the well is 95 ft. The depth of the pond is unknown. The tritium concentrations of the surface water sample and the sample from the well are 2.14 and 1.64 TU, respectively. Both are within the range of post-bomb tritium values. In western Lafayette Parish, near the Town of Duson, well LF-DL-1 is located approximately 150 ft to the west of a Type III landfill (Figure 16). The landfill, which was permitted in 2004 by the Louisiana Department of Environmental Quality, is the site of an abandoned 10-acre borrow pit which had been excavated to a depth of 60 ft. In 2002, LBG-Guyton conducted field surveys of the pit to support the owner’s application to secure a license to operate the landfill. LBG-Guyton observed that the pit retains water after heavy rainfall. Water does not appear to discharge by seepage through the beds of clay and silty clay which form the base of the pit. Dessication cracks and dry sediments within two feet of the base of the pit were interpreted to indicate that evaporation is the principal means by which water is removed from the pit. The depth of the well is 185 ft, and the tritium concentration associated with the sample from the well is 0.06 TU. Radiocarbon The radiocarbon values (Table 2, Figure 17) range from a high of 108 PMC (LF-CC-3) to low of 43.25 PMC (LF-DL-1), with a median of 70.45 PMC (LF-CC-5). The oldest apparent age (based on a straightforward application of the radiometric decay equation (that is, assuming 100 PMC for the value of Ao) is 6,730 years. The 108 PMC of LF-CC-3 indicates recent recharge. The median PMC yields a calculated apparent age of 2,810 years. With regard to municipal supply wells owned by LUS, the apparent ages range from 6,040 years (LF-LUS-3) to 4,370 years (LF-LUS-1).

10

Final Draft

The relation between PMC and the depths of wells from which the samples were collected is illustrated by Figure 18. The apparent age of ground water appears to increase toward the west (Figure 19). This is consistent with the direction of flow based on the regional potentiometric map (Figure 6). In a confined or semi-confined system, the residence time of ground water should be expected to increase along flow paths inferred from potentiometric contours. As shown by Figure 6, the direction of flow of ground water in Lafayette Parish is westward in response to pumpage in Evangeline, Acadia and Jefferson Davis parishes. Deuterium and Oxygen-18 The 2H and 18O signatures (-values; Table 3) plot parallel with but slightly below the Global Meteoric Water Line (GMWL) (Figure 20). The location of the points with respect to the GMWL indicates that the source of the ground waters is precipitation. There does not, at first, appear to be a high degree of homogeneity with -values reported by the USGS for Lafayette Parish and other parishes which overlie the Chicot aquifer (Fendick and Tollett, 2004; Tollett and Fendick, 2004). Although the -values from this study are lower than those of the investigations by Fendick and Tollett (2004) and Tollett and Fendick (2004), all of the -values from this investigation and those of the USGS (Fendick and Tollett, 2004; Tollett and Fendick, 2004) are within the expected range of stable isotope ratios of Louisiana precipitation (Figures 21 and 22), as estimated by the Online Isotopes in Precipitation Calculator (OICP; www.waterisotopes.org), developed and maintained by the University of Utah. The OICP calculator estimates stable isotope ratios, based on factors such as latitude, longitude, surface elevation, average monthly precipitation, temperature, and measured -values in precipitation from known sample locations. Discussion of Isotope Data Tritium and Radiocarbon A significant question with respect to the unadjusted radiocarbon ages posted on Figure 19 is: Is there sufficient reason to suspect that the apparent ages are overestimated because of factors which have altered (e.g., reduced) radiocarbon signatures? In order for the radioactive decay equation (eq. 5) to be applied without correction, it is imperative that the only source of change to the radiocarbon signature be decay of carbon-14. Mixing with waters depleted in radiocarbon or the dissolution of old carbonate rocks can reduce PMC enough to yield apparent ages which are thousands of years older than actual ages. Fortunately, tritium and carbon-13 provide two means of determing whether the apparent ages warrant adjustment. Figure 23 illustrates the association between tritium and radiocarbon among the 13 samples. The clustering of points suggests that there are two groups. One consists of seven samples with radiocarbon measurements between 108 and 70 PMC (2,800 years to recent). Six members of this group are clustered around a median of approximately 80 PMC (1,800 years). The range of tritium values is 3.5 to 0.5 TU, and the median is about 1.7 TU. The tritium values are consistent with recharge during the bomb/post-bomb era.

11

Final Draft

The PMC values of the second group are between 61 and 43 PMC (6,040 to 4030 years), with an approximate median of 52 PMC (about 5,300 years). Tritium measurements of this group range from 0.12 TU to zero (or “not detected”). These tritium measurements are interpreted to indicate pre-bomb age dates. With reference to the very different half-lives of tritium and radiocarbon (12.32 and 5,730 years, respectively), it is not possible to cite radioactive decay as the only factor accounting for the signatures of the first cluster of points. The combination of “old” carbon-14 ages with bomb/post bomb tritium points to the dilution of radiocarbon with dead carbon from an inorganic source as a probable explanation for the inconsistent age indicators. A cross plot of radiocarbon measurements and carbon-13 (Figure 24) shows that PMC decreases as ground water becomes enriched in carbon-13. This is consistent with the interaction of ground water with one or more sources of inorganic carbon (e.g., shell material, caliche, limestone). The dissolved carbonate material is also highly enriched in carbon-13. Dead carbon lowers the radiocarbon signature (lowers the PMC value), and this leads to apparent ages which are greater than actual ages. The potential for this to occur is indicated by equilibrium geochemical calculations which yield negative calcite (a major carbonate mineral) saturation indices for nine of the 13 samples (Figure 25). The calculations were made with the geochemical modeling program PHREEQC (Parkhurst and Appelo, 1999). A saturation index (SI) is a measurement of the degree to which a sample of water is either at equilibrium, oversaturated or undersaturated with respect to a specific mineral phase. A value of zero indicates that the system is at equilibrium with the mineral phase. In such cases, the mineral will neither tend to dissolve nor precipitate (e.g., dissolution and precipitation are balanced). Positive SI values indicate a state of oversaturation, and a tendency for the mineral to precipitate from solution. Negative SI values indicate undersaturation, and the potential for the mineral to dissolve if it is present in the system. Nine of the SIs calculated by PHREEQC indicate that calcite, if it is present in the system, will tend to dissove. The positive SIs indicate a tendency for the precipitation of calcite. Adjusting Ao The above data indicate that the PMC values require adjustment in order to account for the effects of dilution. If the approximate median and low PMCs of the first group from Figure 18 are used as estimates of Ao, the adjusted radiocarbon ages are lowered accordingly. The apparent and adjusted ages, along with the associated tritium values, are listed in Table 2. With reference to the apparent radiocarbon ages, only one well (LF-CC-3) yields a “recent” ground water age. Resetting the value of Ao to 80 PMC has the effect of reducing all age estimates, such that three additional wells (LF-YV-1, LF-DB-1, LF-TD-1) yield recent ages. If 70 PMC is used as the value of Ao, the ages are again lowered, and two other wells (LF-VC-1 and LF-ND-1) are listed as having recent ages. In this case, the major supply wells of the Lafayette Utilities System (LF-LUS-1, LF-LUS-3, LF-LUS-4, and LF-LUS-12), the City of Carencro (LF-CC-4), and the Derouen Landfill (LF-DL-1) have ages indicating long residence time (e.g., greater than 1,000 years). The water supply wells and the landfill monitoring well are among the deepest of the wells sampled for this investigation. It is LBG-Guyton’s opinion that the estimates based on Ao of 70 PMC represent

12

Final Draft reasonable adjusted ages of Chicot aquifer ground waters. These estimates are more consistent with the tritium concentrations of this investigations as well as the CFC ages of shallow Chicot aquifer waters (Fendick and Tollett, 2004; Tollett and Fendick, 2004). Deuterium and Oxygen-18 The -values of Lafayette Parish ground waters and other parishes of southwestern Louisiana are consistent with those of Holocene (~ the last 10,000 years) precipitation of the central Gulf Coast. The homogeneity indicates that waters which have charged the sands of the aquifer are traceable to precipitation within the span of time during which the climate of the Gulf Coast evolved to its present state. Precipitation in the cooler and wetter pre-Holocene climate would likely have been more depleted than modern precipitation in both deuterium and oxygen-18. Thus, the ground water -values indirectly eliminate pre-Holocene precipitation as the source of water, while suggesting precipitation probably within the later period of Holocene time. Conceptual Model of Recharge and Comments on Areas of Concern The CFC ages reported by the USGS (Fendick and Tollett, 2004; Tollett and Fendick, 2004) and the tritium and adjusted radiocarbon data from this investigation support a conceptual model of recharge in which the vertical movement of water through the surface layer of the Chicot in most areas of Lafayette Parish is on the order of three or more decades. This indicates that water does not move rapidly through the surface formations in most areas of the parish to become the principal source of ground water of the Upper sand of the Chicot or to cause concern that contaminants spilled on the surface are immediate threats to the quality of drinking water for customers of the LUS. Ground waters of the Upper and Lower sands of the Chicot are older (by 1,000 years or more) and appear to be derived principally from lateral inflow - not vertical leakage through the surface formations which overlie the aquifer in Lafayette Parish. The data in Table 2 suggest that the four municipal supply wells owned by the LUS are not likely to be affected by surface or near-surface contaminants, but that shallower public supply wells in areas to the north (ND-1, CC-3, and CC-5) of Lafayette appear to be more vulnerable to contamination than the deeper wells owned by the LUS. The supply wells north of Lafayette are either in or near the François Coulee drainage network, a feature which has been postulated to be a zone of higher recharge potential than other areas of the parish (Williams, 1996). Based on this survey, LBG-Guyton infers that this area of north Lafayette Parish provides a more direct pathway for the infiltration of water to the uppermost sands of the Chicot aquifer than most other areas of the parish; and LBG-Guyton recommends that administrators of the North District and the City of Carencro develop and enforce strict wellhead protection guidelines to minimize the potential for contamination of ground water in the vicinity of the well fields. Future water supply wells in this area should be drilled to, and screened within, the Lower sand. Within most areas of the city limits of Lafayette, the formations which overlie the Chicot appear to be a more effective aquitard than in areas north of the city. In similar fashion, the cap in the westernmost areas of the parish appears to be an effective barrier to rapid infiltration of meteoric water as indicated by the combined low tritium concentration and adjusted radiocarbon age (~4,900 years) associated with LF-DL-1. There is at least one area of concern within the boundaries of the City of Lafayette. A

13

Final Draft ground water sample from the supply well at the visitors center (LF-VC-1) yielded 2.62 TU (the second highest of the investigation) and a “recent” adjusted radiocarbon age, based on a value of 70 PMC for Ao. The robust tritium value of this sample is clearly indicative of bomb/post-bomb recharge. Data based on samples from the public supply well at Youngsville (LF-YV-3) also suggest greater potential for contamination than in Lafayette. The adjusted recent radiocarbon age and the tritium analysis (0.49 TU) indicate bomb/post-bomb recharge age dates. The Youngsville well is one of the shallowest of the municipal supply wells sampled for this investigation. Samples from the two private supply wells to the east of Lafayette (LF-DB-1 and LF-TD-1) yielded bomb/post-bomb tritium values. Apparent radiocarbon ages for these wells are 700 and 1,180 years, respectively; but the adjusted ages for both wells are recent. As two of the shallowest wells in the study, both wells appear to tap sands which are hydraulically well connected with the surface. Recommendations The above data indicate a high degree of variability with respect to the recharge potential of the Chicot aquifer in Lafayette Parish. The interpretation, however, is limited by the widespread distribution of wells in the study. The LUS should consider selecting a set of wells in other areas of the parish for sampling and analysis of tritium, radiocarbon, carbon-13, deuterium, and oxygen-18, as this program will provide the information needed to develop a more complete picture of the potential for recharge and contamination of ground water. The additional data will also add to the general conceptual model needed to guide the LUS and other public water suppliers in the parish in the future development and protection of ground water resources. Do not hesitate to contact Dr. Bruce K. Darling (337-257-0206) if you have questions regarding the results, interpretation, or recommendations based on data from this investigation.

Bruce K. Darling, Ph.D. Geochemist/Hydrogeologist LBG-Guyton Associates

14

Final Draft

REFERENCES C.H. Fenstermaker & Associates, LBG-Guyton Associates, The Onebane Law Firm, and HydroEnvironmental Technology (2002), Assistance in Developing a Statewide Ground Water Management Plan, Vol. I - III. Consulting report prepared for the Louisiana Ground Water Management Commission. Craig, H. (1961), Isotopic variations in meteoric waters. Science Vol. 33, pp 1702 -1703. Darling, B.K. (1997), Delineation of the Ground-Water Flow Systems of the Eagle Flat and Red Light Basins of Trans-Pecos, Texas. Ph.D. thesis, Department of Geological Sciences, The University of Texas at Austin, Austin, Texas. Drever, J.I. (1988), The Geochemistry of Natural Waters. Prentice Hall, Inc., New York. Elkins, J.W. (1999), Chlorofluorocarbons (CFCs), in The Chapman & Hall Encyclopedia of Environmental Science. D.E. Alexander and Rhodes W. Fairbridge, eds., pp 78 – 80, Kluwer Academic, Boston, MA. Faure, G., (1991), Principles and Applications of Inorganic Geochemistry. Macmillan Publ. Co., New York. Fendick, R.B. Jr., and Tollett, R.W. (2004), Quality of water from shallow wells in urban residential and light commercial areas in Lafayette Parish, Louisiana, 2001 through 2002. U.S. Geological Survey Water-Resources Investigations Report 03-4118. Fontes, J.-Ch., and Garnier, J.-M. (1979), Determination of the initial 14C activity of total dissolved carbon: a review of existing models and a new approach. Water Resources Research, v. 15, p. 399-413. International Atomic Energy Agency, Global network of isotopes in precipitation data base (http://isohis.iaea.org). Jones, P.H., Hendricks, L.E., and Irelan, B. (1954), Water Resources of Southwestern Louisiana. U.S. Geological Survey Open-File Report prepared in cooperation with the Louisiana Dept. of Public Works and the Louisiana Geological Survey. Lovelace, J.K. (1999), Distribution of salt water in the Chicot aquifer system of southwestern Louisiana, 1995 – 1996. U.S. Geological Survey Water-Resources Investigations Report 014128. Mazor, E. (1997), Chemical and Isotopic Groundwater Hydrology - The Applied Approach: Marcel Dekker, Inc., New York, 413 p.

15

Final Draft Muller, A.G., and Mayo, A.L. (1986), 13C variation in limestone on an aquifer-wide scale and its effecs on groundwater 14C dating models. Radiocarbon, V. 28, no. 3, p. 1041-1054. Parkhurst, D.L., and Appelo, C.A.J. (1999), User’s guide to PHREEQC (version 2) – a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. U.S. Geological Survey Water-Resource Investigations Report 99-4299. Pearson, F. J. (1965), Use of 13C/12C ratios to correct radiocarbon ages of material initially diluted by limestone. In Chatters, R., ed., Proceedings of the 6th International Conference on Radiocarbon and Tritium Dating. Pullman, Washington, U.S. Atomic Energy Commission: 357-366. Tamers, M. A. (1967), Radiocarbon ages of groundwater in an arid zone unconfined aquifer. In Isotope Techniques in the Hydrologic Cycle. American Geophysical Union Monograph 11: 143-152. Tollette, R.W., and Fendick, R.B. (2004), Quality of water from shallow wells in the rice-growing region of southwestern Louisiana, 1999 - 2001. U.S. Geological Survey Water-Resources Investigations Report 03-4050. University of Miami Tritium Laboratory (http://www.rsmas.miami.edu/groups/tritium) Vogel, J.C. (1970), Carbon-14 dating of groundwater, in Isotope Hydrology 1970. International Atomic Energy Agency, Vienna, pp 225 – 239. Wigley, T.M.L. (1975), Carbon-14 dating of groundwater from closed and open systems. Water Resources Research, v. 11, p. 324-328. Williams, C.B. (1996), A hydrogeological study of the Chicot aquifer in Lafayette Parish, Louisiana. Unpublished M.S. thesis in geology, the University of Southwestern Louisiana.

16

Table 1. Wells and Pond Sampled in Conjunction with Age-Dating Study Well #

Latitude & Longitude

Location

Well Depth (ft)

Hydrogeologic Zone

N30º 20.70’ LA 1252 & T-Frere 125 Upper Chicot W 91° 54.93. Rd. N30º 13.67’ Corner of Simcoe and 2 LF-LUS-1 254 Upper Chicot W 91° 54.93. Chestnut Sts. LF-LUSN30º 13.67’ Corner of Simcoe and 567 Lower Chicot 12 W 91° 54.93 Chestnut Sts. N30º 10.57’ LF-LUS-3 810 Broussard Rd. 547 Lower Chicot W 91° 05.98’ N30º 10.57’ LF-LUS-4 810 Broussard Rd. 400 Upper Chicot W 91° 05.98 N30º 13.92’ Lafayette Visitors 3 LF-VC-1 138 Upper Chicot W 91° 58.99 Center, I-49 N30º 19.37’ 600 Blk of Hector 4 LF-CC-3 100 Upper Chicot W 92° 01.03 Connolly Rd N30º 19.14’ 200 Blk of Adrian St. LF-CC-4 452 Lower Chicot W 92° 02.70 Carencro, LA N30º 19.14’ 200 Blk of Adrian St. LF-CC-5 270 Upper Chicot W 92° 02.70 Carencro, LA N30º 06.11’ 5 LF-YV-3 Youngsville, LA 97 Upper Chicot W 91° 59.52 N30º 19.32’ 1300 Blk of Mississippi 6 LF-DB-1 95 W 91° 59.98 Beau Bassin Rd. Alluvium N30º 19.32’ 1300 Blk of 6 LF-DBP-1 Pond Pond W 91° 59.98 Beau Bassin Rd. N30º 18.24’ Mississippi 7 LF-TD-1 339 St. Clair Rd. 65 W 91° 58.44 Alluvium N30º 13.00’ 525 Rue Le Pere 8 LF-DL-1 185 Upper Chicot W 92° 13.58 Duson, LA 1 ND – North Water District 2 LUS – Lafayette Utilities System 3 VC – Lafayette Visitors Center (well is owned by LUS) 4 CC – City of Carencro 5 YV – Town of Youngsville 6 DB and DBP – David Bernard (privately owned well and pond) \7 DAL – Tom Dailey (privately owned well) 8 DLF – Derouen Land Fill (Privately owned well) 1

LF-ND-1

Screen Depth (ft) 115-125 217 – 253 463 – 565 445 – 546 297 – 400 128-138 90 – 100 >400 >200 87 – 97 85 – 95 Pond 55 – 65 175 – 185

Table 2. Age-Dating Results 14C Age1 14C Age2 Well # PMC LF-ND-1 77.25 2,070 281 LF-LUS-1 58.02 4,370 2,580 LF-LUS-12 54.99 4,800 3,011 LF-LUS-3 47.13 6,040 4,250 LF-LUS-4 48.62 5,790 4,000 LF-VC-1 78.90 1,900 111 LF-CC-3 108.50 Recent Recent LF-CC-4 60.52 4,030 2,242 LF-CC-5 70.45 2,810 1,021 LF-YV-3 82.31 1,560 Recent LF-DB-1 91.61 700 Recent LF-TD -1 86.30 1,180 Recent LF-DL-1 43.25 6,730 4,940 1 Ao = 100 PMC; 2Ao = 80 PMC; 3Ao = 70 PMC

14C

Age3 Recent 1,508 1,939 3,178 2,928 Recent Recent 1,169 Recent Recent Recent Recent 3,868

3H

(TU) 1.56 0.13 0.02 0.12 ND 2.62 3.45 0.05 1.85 0.49 1.64 1.84 0.06

Table 3. 2H and 18O in Ground Water and Surface Water – Lafayette Parish Well 2H LF-ND-1 -4.4 LF-LUS-1 -4.4 LF-LUS-12 -4.6 LF-LUS-3 -4.6 LF-LUS-4 -4.6 LF-VC-1 -4.2 LF-CC-3 -4.4 LF-CC-4 -4.5 LF-CC-5 -4.3 LF-YV-3 NS* LF-DB-1 -4.7 LF-DBP-1 -4.1 LF-TD -1 -4.2 LF-DL-1 -4.0 *NS = Not Sampled

18O -30.0 -27.5 -28.5 -29.0 -28.5 -28.0 -29.0 -29.5 -27.5 NS -29.5 -32.0 -28.0 -28.0

Figure 1 Areal Extent of the Chicot Aquifer in Southwestern Louisiana

Aerial Extent of the Chicot Aquifer; Southwestern Louisiana

Rapides

Ve r n on

Allen

Beauregard

Calcasieu

5 10

20

30

40 Miles

Evangeline St. Landry

Jefferson Davis

Cameron

0

·

·

Acadia

St. Martin Lafayette Iberia

Ve r m i li on

St. Martin St. Mary

EXPLANATION Parishes

Water Bodies

Chicot Aquifer

LBG-Guyton Associates

Figure 2 Recharge Potential of Chicot Aquifer

Areas of Recharge Potential: Chicot Aquifer System EXPLANATION Chicot Aquifer

Low Recharge Potential

Moderate Recharge Potential High Recharge Potential

R R aa pp ii dd ee ss V V ee rr nn oo nn

A A ll ll ee nn

B B ee aa uu rr ee gg aa rr dd

S S tt .. LL aa nn dd rr yy E E vv aa nn gg ee ll ii nn ee

S S tt .. LL aa nn dd rr yy

C C aa ll cc aa ss ii ee uu

JJ ee ff ff ee rr ss oo nn D D aa vv ii ss

C C aa m m ee rr oo nn

A A cc aa dd ii aa

S S tt .. M M aa rr tt ii nn LL aa ff aa yy ee tt tt ee

V V ee rr m m ii ll ii oo nn

II bb ee rr ii aa S S tt .. M M aa rr yy

· 0

10

20

40 Miles

Data Source: 'Recharge Potential of Louisiana Aquifers, Geographic NAD83, LDEQ (1999) [aquifer]'


Figure 3 Hydrogeologic North-South Cross Section Through the Chicot Aquifer

Hydrogeologic North-South Cross Section Through the Chicot Aquifer SECTION LINE

CHICOT AQUIFER

Idealized hydrogeologic section through southwestern Louisiana (Lovelace, 1999) From: LaDOTD/USGS Water Resources Technical Report 66


Figure 4 Hydrogeologic East-West Cross Section Through the Chicot Aquifer

Hydrogeologic West-East Cross Section Through the Chicot Aquifer

SECTION LINE

CHICOT AQUIFER

Idealized hydrogeologic section through southwestern Louisiana (Lovelace, 1999) From: LaDOTD/USGS Water Resources Technical Report 66


Figure 5 Location of François Coulee Drainage Network in Lafayette Parish, Louisiana

Location of François Coulee Drainage Network in Lafayette Parish, Louisiana

Source: A hydrogeological study of the Chicot aquifer in Lafayette Parish, Louisiana (Williams, 1996)

Figure 6 Potentiometric Surface – Chicot Aquifer System

Potentiometric Surface Chicot Aquifer System

Rapides Vernon 120' ' 160 ' 140 120' 100'

80'

60' 40' 20'

Beauregard

-20

-4 0

'

0'

20'

'

0 -6

Allen

'

60' 20'

0'

St. Landry

-4 0 -6 ' 0'

-2 0

'

40'

0 -2

Evangeline

0'

80'

120' 100'

'

-40

Calcasieu

-6 0

Jefferson Davis

'

'

Acadia

St. Martin 0'

Lafayette

' -60 -40

Cameron

'

-20'

0'

0

10

20

Potentiometric Surface (ft)

Data Source: U.S. Geological Survey Louisiana Ground-Water Map No. 12 Dated June, 2000

0'

St. Mary

40

Key to Features Chicot Aquifer

Iberia

Vermilion

Miles

³

Note: Elevations are referenced to Mean Sea Level


Figure 7 Coteau Ridge – Hector Connolly Road Between LF-CC-4 and LF-DB-1

A. View to the East where Hector Connolly Road crosses Coteau Ridge B. View to the West where Hector Connolly Road crosses Coteau Ridge

A

B

Figure 8 Chicot Aquifer Wells with CFC Age Dates

Chlorofluorocarbon (CFC) Ages in Years

Verno n

De Ridder

·

Oakdale

Allen !

eauregard

!

!

Lake Charles

!

44

Welsh

46

! 19

!

46 ! 31

Jennings

! Lake Arthur ! 27

Cameron

!

!

Jefferson Davis ! 59

Mamou

43

37

Iowa

Evangeline

48

21

43

!

26

Ville Platte

!

! Eunice

42 ! !

!

35

! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! !

Port Barre

Opelousas

St. Landry

46

EXPLANATION

Church Point Sunset !

17

A c a d i a 38

! Rayne Crowley

L a f a y e !t t e 41 40

36 !!

25

46

Lafayette

!

!

12 ! !! ! 50

Parishes

Water Bodies

30

St. Martin

Chicot Aquifer

32

Cities & Towns

St. Martinville !

Kaplan

Verm i l io n

29 !

Abbeville

New Iberia

Iberia

Jeanerette

CFC Ages

St. Martin Franklin

· 0

Morgan City

St. Mary 5

10

20

Miles


Figure 9 Tritium (TU) in Precipitation at Ottawa, Canada (1953 – 2000)

Month - Year Aug-99

Aug-97

Aug-95

Aug-93

Aug-91

Aug-89

Aug-87

Aug-85

Aug-83

Aug-81

Aug-79

Aug-77

Aug-75

Aug-73

Aug-71

Aug-69

Aug-67

Aug-65

Aug-63

Aug-61

Aug-59

Aug-57

Aug-55

Aug-53

Tritium Units

Tritium in Precipitation at Ottawa, Canada (1953 – 2000) 10,000

1,000

100

10

1

Figure 10 Tritium in Precipitation at Waco, Texas and Baton Rouge, Louisiana

Tritium in Precipitation at Waco, Texas and Baton Rouge, Louisiana 10,000

Baton Rouge Waco

100

3

H (Tritium Units)

1,000

10

Dec-85

Dec-83

Dec-81

Dec-79

Dec-77

Dec-75

Dec-73

Dec-71

Dec-69

Dec-67

Dec-65

Dec-63

Dec-61

1

Month - Year Source: International Atomic Energy Agency – Global Network of Isotopes in Precipitation Database.

Figure 11 Global Meteoric Water Line with Trajectories Showing Effects of Factors which Affect Deuterium and Oxygen-18 Signatures

Global Meteoric Water Line with Trajectories Showing Effects of Factors which Affect Deuterium and Oxygen-18 Signatures

Figure 12 Lafayette Parish Wells with Tritium and Radiocarbon Data

Lafayette Parish Wells with Tritium and C-14 Data

Sunset

Church Point

LF-ND-1

> LF-CC-4

LF-CC-3

>

LF-CC-5

LF-DB-1

> >

LF-TD-1

>

LF-VC-1 LF-LUS-12

Rayne

LF-DL-1

> >

LF-LUS-1

>

Lafayette

LF-LUS-3

>LF-LUS-4

>

EXPLANATION

·

Parishes

Water Bodies

> Kaplan

0

1.5

LF-YV-3

Wells Cities & Towns 3

6 Miles

Abbeville

New Iberia


Figure 13 Tritium in Ground Water – Lafayette Parish

Lafayette Parish Tritium Data (in Tritium Units)

Sunset

Church Point

>

1.56

3.45 1.64

0.05

>1.85 > >

>

1.84

2.62

0.13

Rayne

> 0.06

>0.02 >

Lafayette

0

>0.12

>

EXPLANATION

·

Parishes

Water Bodies

> Kaplan

0

1.5

0.49


6 Miles

Abbeville

New Iberia


Figure 14 Tritium and Depth of Wells in Lafayette Parish

Depth of Wells (ft)

Tritium and Depth of Wells in Lafayette Parish

0 50 100 150 200 250 300 350 400 450 500 0.0

1.0

2.0 Tritium (TU)

3.0

4.0

Figure 15 Pond Adjacent to LF-DB-1

Well LF-DB-1 and Pond

LF-DB-1

Figure 16 Landfill Adjacent to LF-DL-1

A. Location of LF-DL-1 B. Type III Landfill Adjacent to LF-DL-1

LF-DL-1 Well House

A

B

Figure 17 Radiocarbon in Ground Water – Lafayette Parish

Lafayette Parish % Modern Carbon

Sunset

Church Point

>

77.25

108.5 91.61

60.52

> 70.45> >

>

86.3

78.9

> 54.99 >

58.02

Rayne

43.25

>

Lafayette

47.13

> 48.62

>

EXPLANATION

·

Parishes

Water Bodies

> Kaplan

0

1.5

82.31


6 Miles

Abbeville


Figure 18 Radiocarbon and Depth of Wells in Lafayette Parish

Depth of Wells (ft)

Radiocarbon and Depth of Wells in Lafayette Parish

0 50 100 150 200 250 300 350 400 450 500 110

100

90

80

70

60

Radiocarbon (PMC)

50

40

30

Figure 19 Unadjusted Radiocarbon Ages – Lafayette Parish

Lafayette Parish Unadjusted C-14 Ages in Years

Sunset

Church Point

>

2070

0

4030

> > > 2810

700

>

1180

1900

4370

Rayne

>5430

> 4800 >

Lafayette

6040

> 5790

>

EXPLANATION

·

Parishes

Water Bodies

> Kaplan

0

1.5

1560


6 Miles

Abbeville

New Iberia


Figure 20 Abundances of Deuterium and Oxygen-18 in Ground Water – Southwestern Louisiana

Abundances of Deuterium and Oxygen-18 in Ground Water, Southwestern Louisiana

Deuterium (o/oo SMOW)

0

GMWL Laf - USGS Laf - LBG Acadia Allen Evangeline Jeff Davis St. Landry Vernon

-5 Lafayette Parish – samples collected by the USGS

-10 -15 -20 -25

Lafayette Parish – samples collected by LBG-Guyton

-30 -35 -40 -6

-5

-4

-3

-2

Oxygen-18 (o/oo SMOW)

-1

0

Figure 21 Expected Monthly Abundance of Deuterium in Precipitation – Lafayette, Acadia and Vernon Parishes

Expected Abundance of Deuterium in Precipitation – Southwestern Louisiana 0 -5 -10 -15 -20 -25 -30 -35

Lafayette Parish

Acadia Parish

Vernon Parish

Dec

Nov

Oct

Sept

Aug

Jul

Jun

Apr

Mar

Feb

-40

May

Regional average for Southwestern Louisiana

Jan

Deuterium ( o/oo SMOW)

Source: Online Isotopes in Precipitation Calculator – www.waterisotopes.org

Figure 22 Expected Monthly Abundance of Oxygen-18 in Precipitation – Lafayette, Acadia and Vernon Parishes

Expected Abundance of Oxygen-18 in Precipitation, Southwestern Louisiana 0

Oxygen-18 ( o/oo SMOW)

Source: Online Isotopes in Precipitation Calculator – www.waterisotopes.org

-1 -2 -3 -4 -5 -6

Regional average for Southwestern Louisiana

-7 Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec

Lafayette Parish

Acadia Parish

Vernon Parish

Figure 23 Tritium and Radiocarbon in Ground Water – Lafayette Parish

Tritium and Radiocarbon in Ground Water, Lafayette Parish 4.0

Tritium (TU)

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 40

50

60

70

80

90

Radiocarbon (PMC)

100

110

120

Figure 24 Radiocarbon and Abundance of Carbon-13 in Ground Water

Radiocarbon (PMC) and Carbon-13 (o/oo PDB) in Ground Water, Southwestern Louisiana

Radiocarbon (PMC)

120 100 80 60 40 20 0 -25

-20

-15 Carbon-13 ( o/oo PDB)

-10

-5

Figure 25 Calcite Saturation Indices

Calcite Saturation Index

-2.5

Well

LF-DL-1

LF-YV-3

LF-TD-1

LF-DB-1

LF-CC-5

LF-CC-4

LF-CC-3

LF-ND-1

LV-VC-1

LF-LUS-4

LF-LUS-3

LF-LUS-12

LF-LUS-1

Calcite Saturation Indices

0.5

0.0

-0.5

-1.0

-1.5

-2.0

Conceptual Model of Recharge and Comments on Areas of Concern

13

Recommendations

14

References

15 APPENDIX A – TABLES

Table 1. Wells and Pond Sampled in Conjunction with Age-Dating Study Table 2. Age-Dating Results Table 3. 2H and 18O in Ground Water and Surface Water – Lafayette Parish APPENDIX B – FIGURES Figure 1. Areal Extent of the Chicot Aquifer in Southwestern Louisiana Figure 2. Recharge Potential of Chicot Aquifer Figure 3. Hydrogeologic North-South Cross Section through the Chicot Aquifer Figure 4. Hydrogeologic East-West Cross Section through the Chicot Aquifer Figure 5. Location of François Coulee Drainage Network in Lafayette Parish Louisiana Figure 6. Potentiometric Surface – Chicot Aquifer System Figure 7. Coteau Ridge – Hector Connolly Road between LF-CC-3 and LF-DB-1 Figure 8. Chicot Aquifer Wells with CFC Age Dates Figure 9. Tritium (TU) in Precipitation at Ottawa, Canada (1953 – 2000) Figure 10. Tritium in Precipitation at Waco, Texas and Baton Rouge, Louisiana Figure 11. Global Meteoric Water Line with Trajectories Showing Effects of Factors which Affect Deuterium and Oxygen-18 Signatures Figure 12. Lafayette Parish Wells with Tritium and Radiocarbon Data Figure 13. Tritium in Ground Water – Lafayette Parish

Figure 14. Tritium and Depth of Wells in Lafayette Parish Figure 15. Pond Adjacent to LF-DB-1 Figure 16. Landfill Adjacent to LF-DL-1 Figure 17. Radiocarbon in Ground Water – Lafayette Parish Figure 18. Radiocarbon and Depth of Wells in Lafayette Parish Figure 19. Unadjusted Radiocarbon Ages – Lafayette Parish Figure 20. Abundances of Deuterium and Oxygen-18 in Ground Water – Southwestern Louisiana Figure 21. Expected Monthly Abundance of Deuterium in Precipitation – Lafayette, Acadia and Vernon Parishes Figure 22. Expected Monthly Abundance of Oxygen-18 in Precipitation – Lafayette, Acadia and Vernon Parishes Figure 23. Tritium and Radiocarbon in Ground Water – Lafayette Parish Figure 24. Radiocarbon and Abundance of Carbon-13 in Ground Water Figure 25. Calcite Saturation Indices

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