Characterizing submarine groundwater discharge: A seepage meter

GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 6, 1297, doi:10.1029/2002GL016000, 2003

Characterizing submarine groundwater discharge: A seepage meter study in Waquoit Bay, Massachusetts Holly A. Michael, Jonathan S. Lubetsky, and Charles F. Harvey Parsons Lab, CEE, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA Received 29 July 2002; revised 8 October 2002; accepted 27 December 2002; published 22 March 2003.

[1] A seepage meter study was performed in Waquoit Bay on Cape Cod, Massachusetts to characterize the amount, pattern, and origin of submarine groundwater discharge. Measurements from grids of 40 seepage meters provide a detailed representation of groundwater flux in both space and time. At the head of the bay, a distinct band of high, saline discharge was observed between 25 and 45 m from the shoreline. Slug tests indicated no pattern of permeability to explain the band of discharge, and the band was not observed offshore of an island where freshwater discharge is negligible. Experiments using clusters of seepage meters showed large variability in discharge at the meter scale and similar temporal variation throughout the domain, reflecting tidal influence primarily near shore. The small-scale variability challenges the assumption of locally homogeneous flow used in many models, and the band of discharge contradicts predictions that total outflow is largely INDEX fresh and decreases monotonically from shore. T ERMS : 1829 Hydrology: Groundwater hydrology; 4235 Oceanography: General: Estuarine processes; 1894 Hydrology: Instruments and techniques; 4825 Oceanography: Biological and Chemical: Geochemistry. Citation: Michael, H. A., J. S. Lubetsky, and C. F. Harvey, Characterizing submarine groundwater discharge: A seepage meter study in Waquoit Bay, Massachusetts, Geophys. Res. Lett., 30(6), 1297, doi:10.1029/ 2002GL016000, 2003.

1. Introduction [2] Coastal groundwater systems have been increasingly studied due to the potential for transport of anthropogenic contaminants and nutrients to coastal waters. Discharging groundwater may adversely affect coastal ecosystems [e.g. Johannes, 1980; Simmons, 1992] and similarly, cycling and intrusion of seawater may influence the geochemistry of the subsurface [Kohout, 1960]. Current understanding of coastal groundwater dynamics is largely derived from analytic and numerical models [e.g., Reilly and Goodman, 1985] that are commonly two-dimensional and lack representation of small-scale variations in flow. Such models predict primarily freshwater discharge that decreases monotonically with distance from shore. Several seepage meter studies [e.g., Bokuniewicz, 1980; Robinson and Gallagher, 1999] appear to support such a pattern. [3] Marine groundwater discharge over large areas has typically been estimated by hydrologic water balances and recently by environmental tracers such as radon, barium, and radium. Seepage meters can accurately measure local groundwater discharge or inflow when fluid flux across the Copyright 2003 by the American Geophysical Union. 0094-8276/03/2002GL016000$05.00

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sediment interface is relatively large [Shaw and Prepas, 1989]. When flows are small, seepage meters may overestimate flux, perhaps due to effects of currents and waves [Shinn et al., 2002], although underestimation has also been reported [Belanger and Montgomery, 1992]. The waves and currents at our study site in Waquoit Bay are minimal, and measured flow rates are up to three orders of magnitude greater than those described in Shinn et al. [2002]. A recent study [Burnett et al., 2002] found that total discharge measured by seepage meters agreed with estimates derived from natural tracers (Ra, Rn), although the tracer method does not describe the spatial pattern or salinity of discharge. [4] The objective of this study is to accurately characterize the rate, pattern, and variability of submarine groundwater discharge using a dense field of seepage meters.

2. Field Measurements 2.1. Study Site: Waquoit Bay [ 5 ] The Waquoit Bay National Estuarine Research Reserve is located on the southern shore of Cape Cod, Massachusetts. This coastal embayment (Figure 1) is approximately 3 km2 in area with an average depth of 1 m, and has been the subject of several previous studies [e.g., Valiela et al., 1990; Charette et al., 2001]. The Cape Cod aquifer is unconfined and 100– 120 m thick in this area. It includes an upper permeable layer, approximately 11 m thick, which is underlain by a less permeable layer of fine sand, silt, and clay, and bounded below by basal till and bedrock. The head of the bay sub-watershed is 0.76 km2 in area, with a recharge rate of about 46 cm/yr, and an estimated hydraulic gradient of 0.002 [Cambareri and Eichner, 1998]. 2.2. Seepage Meter Construction and Sampling [6] Forty seepage meters were constructed from the ends of 55-gallon drums similar to those described in Lee [1977]. Each has a 7.5 cm vent hole, left open during placement so that pressure quickly equilibrated with the bay. Discharging water was collected in a thin-walled plastic bag attached with a quick-connect fitting. Each bag was pre-filled with 1L of bay water to prevent underfilling [Shaw and Prepas, 1989] and to allow for measurement of flow into the sediments. After deployment, the bags were weighed to determine the amount of groundwater seepage and salinity was measured. 2.3. Discharge Patterns 2.3.1. Head of the Bay [7] Two sampling campaigns were conducted at the head of Waquoit Bay during August 1999 and July 2000. Both used 40 seepage meters arrayed in four transects perpendicular to the coast (Figure 1). These were sampled every two hours over a complete tidal cycle. A distinct band of - 1

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MICHAEL ET AL.: CHARACTERIZING SUBMARINE GROUNDWATER DISCHARGE

Figure 2. (a) and (b) Time-averaged groundwater discharge vs. distance separated into freshwater, saltwater, and unknown components for the head of the bay experiments. (c) and (d) Corresponding standard deviation in space and time. Figure 1. Map of Waquoit Bay: experimental design for seepage meter studies. high discharge parallel to the coast between 20 and 45 m from the shore (Figures 2a, 2b, and 3a) was observed in all four transects and six time intervals in both campaigns. Inflow was measured in 15 samples in 1999 and 7 in 2000. This inflow occurred primarily in seepage meters located far from shore, with no apparent correlation to the bay water level over single tidal cycles. [8] The proportion of freshwater discharging into the seepage meters was calculated from the salinity and volume of the discharge and the estimated salinity of the baywater that recharges the sediments, which may vary over time. An upper bound for the salinity of the inflowing baywater may be estimated as 33.0 ppt, the salinity of the seawater just outside the mouth of Waquoit Bay in Vineyard Sound, and the lower bound as the highest salinity among all the seepage meter bags: 29.3 and 30.6 ppt for 1999 and 2000, respectively. This uncertainty results in the small ‘unknown’ component of outflow depicted in Figures 2a and 2b. However, most of the discharging water was saline with the exception of some fresher discharge in the row of meters nearest the shore. [9] Slug tests conducted at 6 locations along a transect into the bay indicated that the permeability of bay sediments generally decreases with distance from shore (Figure 3b). Piezometers screened over the bottom 0.2 m were driven 0.6 m into the sediment. A slug of water was added to the clear PVC top section of the piezometers and the water level was recorded as it dropped. The permeability relative to the location nearest to shore was estimated by normalizing the inverse of the time for the water level to drop 90% of the distance to the bay surface by the inverse of the time in the piezometer nearest shore. 2.3.2. Minimal Freshwater Flow: Island Study [10] A third seepage meter experiment was conducted on a narrow piece of land jutting off of Washburn Island (Figure 1) in August 2000 to investigate the groundwater discharge pattern where there is little freshwater. Freshwater flow is small because this narrow spit drains a very small

area, thereby virtually eliminating density differences and a regional gradient while maintaining tidal forces. The twenty seepage meters in this study were sampled every 2 hours over a tidal cycle with a range of 0.56 m. They reveal saline discharge that is essentially uniform and ranges from 0.11 to 0.22 m/d when averaged over time, contrasting sharply with the pattern at the head of the bay. The seepage variation that we do see likely results from the natural spatial variability observed in all seepage meter experiments, and the nonzero total discharge may be caused by tidal pumping, or a small head gradient due to tidal currents around the spit of land. 2.4. Spatial and Temporal Variability 2.4.1. Head of the Bay Experiments, 50 m Scale [11] The seepage meter grids indicate large variability in discharge over both time and space (Figures 2c and 2d). The spatial standard deviation as a function of distance into the bay was calculated from the four seepage meters in each row after averaging the data from each seepage meter over time. Similarly, the temporal standard deviation was calcu-

Figure 3. (a) Grayscale, contours are time-averaged groundwater discharge [m/d] for the 2000 head of the bay experiment. (b) Normalized permeability as a function of distance into the bay. Triangles and circles are approximate slug test and seepage meter locations.


lated over the six time periods after averaging the discharge across the four seepage meters in each row. Comparing these plots to the plots of discharge vs. distance from shore (Figures 2a and 2b) shows that greater discharge correlates with greater variability. 2.4.2. Cluster Experiments, 1 m Scale [12] Two further experiments were conducted with clusters of seepage meters spaced closely together to characterize variability over smaller areas and longer times. In the 1999 cluster experiment, nine seepage meters were placed in the nearshore zone (Figure 4) and sampled during daylight hours for two-hour periods on six days over two weeks. The 2001 experiment examines discharge variability on both a large (50 m) and small (1 m) scale. Eighteen seepage meters arranged in clusters along one transect (Figure 1) were sampled every two hours over three tidal cycles, except for the far from shore meters which were not sampled overnight. [13] Data from the cluster experiments reveal that differences in discharge over small spatial scales (1 m) can be similar in magnitude to differences in discharge over larger spatial scales. The 1999 cluster experiment (Figure 4) indicates that seepage meters located next to each other may differ greatly in discharge. For example, during the same two-hour period, two seepage meters less than 2 m apart registered 0.05 m/d and 0.37 m/d. [14] In the 2001 cluster experiment, the standard deviation of the time-averaged data is 0.029 m/d for the seepage meters in the nearshore cluster and 0.053 m/d for the middle zone cluster. All of the data taken together, spanning nearly 60 m, also has a standard deviation of 0.053 m/d. [15] The 1999 cluster experiment (Figure 4) indicates that discharge into the bay varies significantly from day to day as well as during a tidal cycle. However, the relative temporal discharge is constant: an area which discharges more than a neighboring area does so steadily, even as the total discharge increases or decreases. The 2001 cluster experiment shows that temporal variation with the tide changes with distance into the bay (Figure 5). The nearshore (0 – 30 m from shore) discharge exhibits a clear inverse variation with the tide. The largest factor of change in this zone occurs during periods of greatest tidal range, and every seepage meter exhibited a similar inverse variation with the tide. The discharge in the middle (30 – 50 m from shore) and far (50 – 70 m from shore) zones does not vary as consistently with the tides, although the middle area may exhibit variation due to a longer-scale driving force such as the spring-neap tidal cycle. Discharge has been shown to correlate with tidal magnitudes over long time scales [Taniguchi, 2002], an observation that is consistent with both the increased discharge in the middle zone as the tidal magnitude increased, and the larger discharge in the 2000 head of the bay experiment relative to 1999.

3. Discussion 3.1. How Many Meters are Necessary? [16] The large variability in discharge raises the question of how many seepage meters are necessary to adequately estimate the large-scale discharge. The estimated total groundwater flux from 1000 replicates of random selections of seepage meters in the 1999 head of the bay experiment indicate an average absolute difference in flux using 10, 20, and 30 seepage meters, as compared to all 40 meters, of 34%,

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Figure 4. Daily average groundwater discharge vs. time for selected seepage meters in the 1999 cluster experiment. Meter 11 is 12 m from shore. Bars represent ± one standard deviation of the measurements taken on each day. 19% and 10%, respectively. However, if seepage meters are arranged in transects, the error is much lower. The flux estimated using one, two, and three transects of 10 seepage meters differs from the flux estimated using all four transects by 9%, 4%, and 3% respectively. Thus, on a 50 m scale, 20 seepage meters arranged in transects appear adequate to estimate total discharge at our site within a reasonable error. 3.2. Discharge Comparison With Freshwater Balance [17] Cambareri and Eichner [1998] estimated the freshwater input to Waquoit Bay from the head of the bay subwatershed to be 0.012 m3/s using a hydrologic balance based on a yearly average precipitation of 92 cm. A conservative extrapolation of our 1999 and 2000 seepage meter data along the 610 m head of the bay results in total discharge estimates of 0.047 and 0.106 m3/s, respectively. These total discharge estimates are much greater than the freshwater estimate, indicating that there is significantly more saline than fresh discharge. The freshwater discharge rate estimates from the 2000 experiment using the upper and lower bounds for recharging salinity are 0.011 and 0.004 m3/s, respectively, and the values from 1999 are 0.006 and 0.001 m3/s. These values are lower than that of the freshwater balance, but consistent given the likelihood that there is significant freshwater discharge nearer to shore than the shallowest seepage meter. [18] The observation that total discharge was a factor of 2.3 larger in the 2000 experiment than the 1999 experiment may be explained by either the larger tidal magnitude in 2000 or the greater precipitation preceding the 2000 experiment. The tidal range during the 2000 experiment was 40 cm (1 day prior to spring tide) twice that of the 1999 experiment (3 days after neap tide). During the three months and 1 year prior to the July 2000 experiment 28.7 cm and 129.3 cm [Payne, 2002] of precipitation fell at Long Pond in Falmouth (7 km from Waquoit Bay). This is significantly more than the 18.9 cm and 93.1 cm that fell prior to the 1999 study. 3.3. Large-Scale Pattern of Discharge [19] Despite the small-scale variability, a band of high discharge following the shoreline is clearly evident. Although, to our knowledge, the banded pattern has not

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groundwater dynamics in sandy coastal aquifers and methods used to investigate discharge. A banded pattern of mostly saline groundwater discharge at the head of Waquoit Bay suggests that flow follows more complex patterns at the coast than models have predicted: the interaction between fresh and saltwater, driven by both tides and freshwater discharge, may create unanticipated circulation cells and flowpaths. This large-scale pattern is only evident when a sufficient density of measurements is used to overcome the small-scale variability. The large differences in flow observed over small spatial scales raise questions about the application of models that assume homogeneity. Lastly, the large proportion of saline discharge may have implications for the use of geochemical tracers to estimate total submarine groundwater discharge if concentrations in fresh and saline water differ. Figure 5. 2001 multiple tidal-cycle experiment. been previously reported in a coastal system, there are studies which provide evidence of higher discharge offshore [Simmons, 1992; Cable et al., 1997]. It is possible that diagenetic changes or recent sedimentation on the bottom could create a high permeability pattern aligned with the shoreline, but slug tests show no such pattern. The uniform discharge observed on Washburn Island suggests that density-dependent flow may play a role in creating the banded discharge pattern. [20] Although our results provide a detailed characterization of discharge patterns, they do not show where the discharging saline water originates. Some seepage meters indicated inflow, but they do not account for the large outflow of saline water. One source of saline water is recharge during a rising tide [Nielsen, 1990] that overtops discharging freshwater and results in an inverse density gradient. A second potential source is offshore circulation of seawater, which was predicted theoretically by Henry in 1959 [Reilly and Goodman, 1985]. Such models predict that saltwater flows into the subsurface far from the shore and circulates toward shore before discharging. Most of our measured inflow occurred in the zone far from shore, which is consistent with the possibility of downwelling further offshore. However, this inflow could not be investigated with seepage meters because the mucky bay floor beyond 75 m from shore prevents stable placement. The question remains whether these mechanisms explain the amount of saltwater discharge and its pattern. [21] Traditional models of submarine groundwater discharge based on simplifying assumptions do not attempt to represent density-driven free convection in which instabilities, closely related to small-scale heterogeneity, may affect larger-scale flow [Simmons et al., 2001]. Recently published 2-D numerical models [Ataie-Ashtiani et al., 1999; Robinson and Gallagher, 1999] show that incorporating tidal dynamics significantly affects salinity distributions and groundwater flow patterns. The data presented here demonstrate both tidal effects and high small-scale variability in flow, raising concerns about models that neglect these factors.

[23] Acknowledgments. We would like to thank the WBNERR staff for their support as well as the many researchers who helped in the field. This work was supported under a National Science Foundation Graduate Research Fellowship and ONR grant #N00014-99-1-0038.

References Ataie-Ashtiani, B., et al., Tidal effects on sea water intrusion in unconfined aquifers, Journal of Hydrology, 216, 17 – 31, 1999. Belanger, T. V., and M. T. Montgomery, Seepage meter errors, Limnol. Oceanogr., 37(8), 1787 – 1795, 1992. Bokuniewicz, H. J., Groundwater seepage into Great South Bay, New York, Estuarine and Coastal Marine Science, 10, 437 – 444, 1980. Burnett, B., et al., Assessing methodologies for measuring groundwater discharge to the ocean, EOS, 83(11), 117 – 122, 2002. Cable, J. E., et al., Magnitude and variations of groundwater seepage along a Florida marine shoreline, Biogeochem., 38, 189 – 205, 1997. Cambareri, T. C., and E. M. Eichner, Watershed delineation and ground water discharge to a coastal embayment, GroundWater, 36(4), 626 – 634, 1998. Charette, M. A., et al., Utility of radium isotopes for evaluating the input and transport of groundwater-derived nitrogen to a Cape Cod estuary, Limnol. Oceanogr, 46(2), 465 – 470, 2001. Johannes, R. E., The ecological significance of the submarine discharge of groundwater, Mar. Ecol. Progr. Ser., 3, 365 – 373, 1980. Kohout, F. A., Cyclic flow of salt water in the Biscayne Aquifer of Southeastern Florida, JGR, 65(7), 2133 – 2141, 1960. Lee, D. R., A device for measuring seepage flux in lakes and estuaries, Limnol. Oceanogr., 22(1), 140 – 147, 1977. Nielsen, P., Tidal dynamics of the water table in beaches, WRR, 26(9), 2127 – 2134, 1990. Payne, R., Falmouth Water Department, www.whoi.edu/climate/, 2002. Reilly, T. E., and A. S. Goodman, Quantitative analysis of saltwater-freshwater relationships in groundwater systems—a historical perspective, Journal of Hydrology, 80, 125 – 160, 1985. Robinson, M. A., and D. L. Gallagher, A model of ground water discharge from an unconfined coastal aquifer, GroundWater, 37(1), 80 – 87, 1999. Shaw, R. D., and E. E. Prepas, Anomalous, short-term influx of water into seepage meters, Limnol. Oceanogr., 34(7), 1343 – 1351, 1989. Shinn, E. A., et al., Seepage meters and Bernoulli’s revenge, Estuaries, 25(1), 126 – 132, 2002. Simmons, G. M., Importance of submarine groundwater discharge and seawater cycling to material flux across sediment/water interfaces in marine environments, Mar. Ecol Progr Ser, 84, 173 – 184, 1992. Simmons, C. T., et al., Variable-density groundwater flow and solute transport in heterogeneous porous media: approaches resolutions and future challenges, Jour of Contam Hydrol, 52, 245 – 275, 2001. Taniguchi, M., Tidal effects on submarine groundwater discharge into the ocean, GRL, 29(12), 10.1029/2002GL014987, 2002. Valiela, I., et al., Transport of groundwater-borne nutrients from watersheds and their effects on coastal waters, Biogeoch, 10, 177 – 197, 1990.

4. Conclusion [22] This study gives specific information about groundwater flow into Waquoit Bay and also provides insight into

H. A. Michael, J. S. Lubetsky, and C. F. Harvey, Parsons Lab, CEE, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.