Ensenada, Baja California, México. aBStract: to evaluate seawater intrusion into a coastal aquifer Sr isotopes and concentrations of water samples from wells of ...
Water-Rock Interaction – Birkle & Torres-Alvarado (eds) © 2010 Taylor & Francis Group, London, ISBN 978-0-415-60426-0
Evaluation of seawater intrusion using Sr isotopes: An example from Ensenada, B.C., México K.M. Lara & B. Weber
Departamento de Geología, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Ensenada, Baja California, México
ABSTRACT: To evaluate seawater intrusion into a coastal aquifer Sr isotopes and concentrations of water samples from wells of the Maneadero valley (Ensenada, Baja California) and from recharge feeder were analyzed. The NE feeder has 87Sr/86Sr of ∼0.7076 that is significantly lower than seawater Sr (∼0.7092). The SE feeder has 87Sr/86Sr as low as ∼0.7063 but higher Sr concentrations. Water samples from the wells have 87Sr/86Sr ranging between the values of these two recharging creeks. Samples from wells close to the coast have the highest salinities and Sr concentrations, respectively, but none of them has 87 Sr/86Sr higher than the NE feeder Sr. Mixing models suggest only minor amount of seawater is added to the aquifer. Hence, high salinities of water from some wells cannot be explained by seawater intrusion only. We suggest increasing salinity by evaporation and salt recycling from irrigation as a reasonable process to explain high salinities. 1 InTroduction
to determine and to quantify sources of recharge (e.g. Jørgensen et al. 2008, Langman & Ellis 2010).
Scientists become increasingly concerned about the effects of seawater intrusion on coastal aquifers. Due to β- decay of 87Rb to 87Sr the 87Sr/86Sr depends on the Rb/Sr ratio and the age of a given rock or mineral. The 87Sr/86Sr of present-day oceans is constant around the world (0.70918 ± 0.00001; Faure & Mensing 2005) due to the relatively long residence time (5.0 × 106 years) of strontium in seawater and its complete homogenization within about 103 years. Considering that the Sr concentration of seawater (7.74 ppm) is about 10–100 times greater than that of average river water, mixing of seawater with continental water (by assuming the Sr isotopic composition of river water being sufficiently different from seawater) will cause an immediate response not only on the Sr concentration but also on its isotopic composition. On the other hand, if salinity of water within a given aquifer rises due to reasons other than seawater intrusion (like evaporation and salt recycling from irrigation, Milnes & Renard 2004) then the Sr isotopic composition in the aquifer will remain unaffected. In samples from coastal aquifers that show relatively high salinity the analysis of element concentrations alone cannot always distinguish between increasing salinity that comes from intruded seawater or from increasing salinity due to evaporation at the surface and salt recycling. Strontium isotopic composition modeling, instead, is a useful tool not only for quantifying the amount of seawater that intruded the coastal aquifer but also
2 Regional background Coastal aquifers provide fresh water all over the world, especially in semiarid and arid zones. Due to their proximity to the ocean, coastal aquifers are highly sensitive to disturbances. Sometimes, when a coastal aquifer is overexploited, the phreatic level lowers below the sea level and, in order to restore the hydrologic balance, marine water moves towards the zones occupied previously by fresh water. According to the CNA (for the Mexican National Water Commission), there are 17 coastal aquifers with seawater intrusion problems known in Mexico. The Maneadero aquifer, that provides fresh water to the city of Ensenada, Baja California, is one of those aquifers, having problems of decreasing water quality due to rising salinity of water in the wells. The Maneadero aquifer supplies about 70% of the water used to sustain extensive agriculture in the Maneadero Valley. However, recharge rates have not kept pace with extraction and they have generally been less than the total discharge of the aquifer (i.e. extraction, evaporation and losses to the ocean). Since 1968, the year after which extraction increased significantly, the discharge rates are higher than recharge rates (Daesslé et al. 2004). Geophysical studies (e.g. Lujan 2006 and references therein) have shown increasing salinity of the Maneadero aquifer over past decades, which
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Either 10, 20, or 50 ml of each water sample were weighed, spiked with a Sr tracer (99.89% 84Sr), and evaporated to dryness. To remove any organic matter, 5–10 drops H2O2 were added to the samples and evaporated again. Then, the samples were dissolved in double distilled 8N HNO3 and loaded onto ion exchange columns packed with ∼300 µl of Sr-Spec® resin. Major and interfering elements were eluted first with 8M, then with 3M, and finally with 0.3M HNO3. The final Sr cut was collected with ultrapure water and dried down in small Teflon® beakers. Chemical and chromatographic procedures were carried out in class 100 cleanlab facilities at the Geology Department, CICESE, Ensenada. Total procedure blanks were typically 0.1 ng/g Sr. Isotope ratios were analyzed with a Finnigan MAT 262 multicollector mass spectrometer at the Geophysics Institute, UNAM, México City by measuring simultaneously 88, 87, 86, 85 (for Rb monitor), and 84 masses on faraday collectors in static mode. Six blocks of ten 16s integrations each block were measured per run. 87Sr/86Sr were corrected for mass fractionation by normalizing to 86Sr/88Sr = 0.1194. Values were adjusted by a –0.0085% correction factor obtained from repeated measurements of the NIST987 standard.
Figure 1. Simplified geologic map of the Maneadero area showing sample sites of wells in the sedimentary basin (white) and from creeks. Igneous rocks (cross filling) are Cretaceous volcanic rocks and granite. Note: Dotted area around estuary is partially flooded when tide is in.
was interpreted in terms of progressive seawater intrusion. The hydrogeochemical evolution of the Maneadero aquifer was studied by Daesslé et al. (2004) during a season that was dryer than average (from 2001 to 2002). These authors concluded that the rise in TDS levels (total dissolved solids) through time indicates the vulnerability of the aquifer to seawater intrusion. The comparison of TDS data from different wells revealed that the areas most affected by seawater intrusion were those close to the coast and in the center of the aquifer. Furthermore, the authors suggested a progression of seawater intrusion towards the wells into San Carlos creek (Fig. 1). In this approach to evaluate and to quantify possible seawater intrusion into the Maneadero aquifer, we applied 87Sr/86Sr ratios and Sr concentrations obtained by isotope dilution analysis.
4 RESULTS Measured conductivities, Sr concentrations obtained by isotope dilution, and isotope ratios of samples from sites shown in Figure 1 are listed in Table 1 and displayed in Figure 2 together with calculated hypothetical mixing lines. Conductivities and major element concentrations (Ca, Na, Mg, K; not shown) of all samples show linear correlations with respect to the Sr concentrations, indicating that Sr concentrations behave proportional to salinity. San Carlos Creek, which carries water around the year, is the northeastern recharge feeder for the Maneadero aquifer (C, E; Fig.1). It has the highest 87Sr/86Sr (0.70757–0.70761) and moderate Sr concentrations (0.3–0.6 ppm). One sample collected from the San Carlos thermal spring (D) has the lowest 87Sr/86Sr (0.70527) and Sr concentration (0.08 ppm) of all analyzed samples, suggesting that thermal springs, which are linked to major active faults, have a different origin. Samples collected from the SE recharge feeder (San Francisquito Creek, Fig. 1) have 87Sr/86Sr of 0.70626 (G, wet) and 0.70659 (G, dry) with significantly higher Sr concentrations of ∼1.8 ppm. Two samples from the southern feeder (Las Animas Creek) that were recently taken after the 2010 raining season plot close to a mixing line calculated from NE and SE feeder creek data (Fig. 2).
3 ANALYTICAL PROCEDURE Water samples were collected from wells throughout the Maneadero Valley (Fig. 1) and from recharge feeder creeks, as well as from a thermal spring (D) at the end of the wet and dry seasons in March and October 2009, respectively. Conductivity was measured on-site, as a first estimation of aquifer salinity. Water samples were collected in pre-cleaned polyethylene bottles, filtered, and gently acidified with a few mL of double distilled HNO3. In order to characterize the chemical composition of the samples and to obtain an estimate of the Sr content to enhance spiking, cation contents were measured with a ICP-OES Liberty 110 at CICESE.
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Table 1. 87Sr/86Sr, Sr concentrations and conductivies of water samples from Maneadero Valley and adjacent creeks. Sample Site
Date
Conduct. mS
Sr ppm
A A B B C C C1* D E E F G G H I J K L M N O Q Q T V
Mar-09 Oct-09 Mar-09 Oct-09 Mar-09 Oct-09 Oct-09 Mar-09 Mar-09 Oct-09 Mar-09 Mar-09 Oct-09 Oct-09 Oct-09 Oct-09 Oct-09 Oct-09 Oct-09 Oct-09 Oct-09 Apr-09 Oct-09 Mar-10 Mar-10
4.51 5.60 1.70 1.89 1.62 1.98 1.98 0.81 1.47 1.44 1.52 4.38 5.44 10.3 45.9 7.33 7.25 9.23 2.72 3.51 3.71 7.42 9.19 2.31 2.43
1.66 1.74 0.56 0.57 0.49 0.54 0.60 0.08 0.45 0.33 0.46 1.82 1.76 2.95 9.54 1.83 2.21 2.71 0.71 1.00 1.00 2.49 2.53 0.73 0.82
Sr/86Sr
87
0.707544 0.707519 0.707324 0.707347 0.707578 0.707574 0.707613 0.705273 0.707598 0.707565 0.707523 0.706259 0.706590 0.706810 0.707284 0.707367 0.706709 0.706751 0.706877 0.706756 0.706735 0.707526 0.707501 0.706996 0.706961
std. err 2σ(m) 13 11 15 15 10 07 09 12 10 08 10 11 10 09 10 10 14 09 28 11 10 08 13 09 15 Figure 2. (a) Sr concentration vs. 87Sr/86Sr and (b) 1/ Sr vs. 87Sr/86Sr diagrams. Stars are water samples from feeder creeks; dots are from wells within Maneadero Valley (see Fig. 1). Grey symbols are samples taken in the dry season, black symbols in the wet season (Note: From some sites samples of both seasons were analyzed.)— Mixing hyperbolas (a) and mixing lines (b) were calculated for mixtures of present-day seawater (87Sr/86Sr = 0.70918, 7.74 ppm Sr; Faure & Mensing 2005; solid black lines) with (1) NE feeder (San Carlos Creek) and (2) with SE feeder (San Francisquito Creek) using sample site G in dry season, which lower 87Sr/86Sr (0.70626) and 1.8 ppm Sr (Note: Seawater composition lies off the diagrams).—Samples from the southern feeder creek, Las Animas (T, V), were taken after heavy rains in spring 2010. Using NE and SE feeder compositions as endmembers, the latter samples plot close to a mixing line (dashed black line) with ~80% NE and ~20% SE.—In addition, black dotted hyperbola illustrates theoretical mixing of San Carlos thermal water (only in Fig. 2a). Grey dashed mixing lines were calculated for mixtures of SE feeder with 30% NE and 50% NE feeder, respectively, which is additionally mixed with seawater (three components).— Heavy grey line (only in Fig. 2b) marks the 87Sr/86Sr value of the higher NE feeder, illustrating that downstream samples and wells have similar isotope ratios with higher Sr concentrations and none of the samples has 87Sr/86Sr above the grey line.
* Sample site C1 close to sample site C, on Fig. 1.
In the northern section of the valley, downstream from San Carlos Creek, samples A, F, and Q have similar Sr isotopic composition to San Carlos Creek (C, E). Sample A and Q, however, have significantly higher Sr concentration with respect to San Carlos Creek. Samples B and J both yielded 87Sr/86Sr slightly below NE feeder values (0.70732–0.70737), of which sample J has a significantly higher salinity and Sr concentration. Another 7 samples were taken from wells within the southern part of Maneadero Valley (Fig. 1). Samples M, N, and O are from wells at a greater distance from the coast. They have relatively low Sr concentrations and 87 Sr/86Sr between 0.70674 and 0.70688. The results of the latter samples plot again close to the NE–SE recharge feeder mixing line (Fig. 2). Samples H, I, K, and L that were taken from wells relatively close to the coast within the main farmed zone of the Maneadero Valley have the highest salinities and Sr concentrations, respectively, ranging from 2.2 to 9.5 ppm Sr (note: average seawater has 7.74 ppm Sr). However, all samples have 87Sr/86Sr ratios between average NE and SE recharge feeder creek
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be considered to explain increasing salinity in the wells of the Maneadero Valley. On the basis of our preliminary data we conclude that seawater intrusion into the coastal aquifer of the Maneadero Valley is not the main reason for increasing salinity of water in the sampled wells. Instead, it is more likely that salinity increases by evaporation at the surface and salt recycling from irrigation (Milnes & Renard 2004), which in turn contaminates the water of the aquifer with Sr having the same isotopic composition as the water from the aquifer itself. Contamination with a minor seawater component is possible only in those wells that are located close to the coast. This interpretation is further confirmed by negative δ18O (–5.2% to –6.9%) and δD (–31% to –42%) values that lie between the SE and NE feeder values for all except one (I) samples analyzed from wells.
data. Samples H, K, and L (0.70671–0.70681) are indistinguishable from mixed samples (M, N, O); the extremely salty (46 mS/ 9.5 ppm Sr) sample “I” has a higher 87Sr/86Sr ratio of 0.70728. 5 DISCUSSION and conclusions In order to evaluate possible seawater intrusion into the aquifer of the Maneadero Valley, we calculated mixing models for seawater and the different potential source compositions analyzed in this study (Fig. 2). None of the analyzed samples from the wells within Maneadero Valley have higher 87 Sr/86Sr ratios than those from the NE recharge feeder (San Carlos Creek). In the northern section of the Maneadero aquifer, where it can be assumed that San Carlos Creek is the only water source, no seawater intrusion is indicated by our data. Most of the samples from the southern part of the valley have significantly lower isotope ratios, indicating another source of water mixed with water from the NE feeder (San Carlos Creek). Another minor water source for the Maneadero aquifer lies at its SE edge. Our only sample site from this area has a lower isotope ratio but a higher Sr concentration with respect to San Carlos Creek. A mixture of both recharge feeder creek waters is indicated for several water samples from wells in the southern part of the aquifer away from the coast, where seawater intrusion is not indicated. Such mixtures of different source waters complicate the evaluation for Maneadero Valley in terms of adequate mixing models, as at least three component mixing needs to be considered. In addition to mixing lines between the different endmembers, theoretical mixing lines of different mixtures of the NE and the SE recharge feeder creek waters with seawater (three component models) are shown in Figure 2. For some samples, such three component mixtures seem to be a reasonable solution, especially for sample J whose composition can be explained by adding ∼8% of seawater to a mixture of about 35% SE and 65% NE recharge feeder creek water. However, such a mixing model is unlikely for samples from site A, which is located north of San Carlos Creek. Contamination of the coastal aquifer with seawater can be estimated for the data from those wells that lie closer to the cost (H, L, K, I, Q), only if a single water source (i.e., from the SE recharge feeder creek) is assumed. In such a scenario samples H, L, and K may contain ∼5% of seawater, whereas samples I and Q may contain more than 10% of seawater (Fig. 2). Considering the intermediate composition obtained from the southern feeder (Las Animas Creek) a three-component mixing should be assumed. Then, seawater contamination must be significantly lower. In view of their relatively high salinities and Sr contents another process than seawater intrusion has to
ACKNOWLEDGEMENTS This study was supported by CICESE internal project 644131. We are grateful to Gabriela Solís Pichardo and Peter Schaaf (both UNAM) for supporting isotope analysis. We want to thank Thomas Kretzschmar for his continuous assessment, Mario Vega for ICP analysis and Gabriel Rendón for lab assistance, all at CICESE. We also thank to Thomas Bullen (USGS) for his comments and Akira Ueda (Kyoto) for review. Thanks to Billy, Ismael, Roberto, and Sarah for their help during fieldwork. references Daesslé, W. Sanchez, E.C., Camacho-Ibar, V.F., MendozaEspinosa, L.G., Carriquiry, J.D., Macias, V.A. & Castro, P.G. 2005. Geochemical evolution of groundwater in the Maneadero coastal aquifer during a dry year in Baja California, Mexico. Hydrogeology Journal 13: 584–595. Faure, G. & Mensing, T.M. 2005. Isotopes: Principles and Applications (Third Edition), John Wiley & Sons, Inc. Jørgensen, N.O., Andersen, M.S. & Engesgaard, P. 2008. Investigation of a dynamic seawater intrusion event using strontium isotopes (87Sr/86Sr). Journal of Hydrology 348: 257–269. Langman, J.B. & Ellis, A.S. 2010. A multi-isotope (δD, δ18O, 87Sr/86Sr, and δ11B) approach for identifying saltwater intrusion and resolving groundwater evolution along the Western Caprock Escarpment of the Southern High Plains, New Mexico. Applied Geochemistry 25: 159–174. Lujan, B. 2006. Utilización de ondas electromagnéticas para detectar la invasión de agua marina en el acuífero del Valle de Maneadero en Ensenada, BC. Tesis MC, CICESE, Ensenada BC, México. Milnes, E. & Renard, P. 2004. The problem of salt recycling and seawater intrusion. in coastal irrigated plains: an example from the Kiti aquifer (Southern Cyprus). Journal of Hydrology 288: 327–343.
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