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Submarine Ground-water Discharge

Submarine Ground-water Discharge Diagram
 

Indian River Lagoon

Quantifying Submarine Groundwater Discharge to Indian River Lagoon, Florida

U.S. Geological Survey Open-File Report 00-492
Download this report as a PDF

Photo taken at study site.

Figure 1. Sampling for submarine ground discharge in Indian River Lagoon.

Introduction

The discharge of submarine ground water has recently been shown to be an important process in many environmentally fragile coastal ecosystems. However, groundwater discharge into coastal bottom water is still an often-overlooked component of many hydrologic and oceanic models.

The exchange of interstitial water across the sediment/water interface may introduce anthropogenic pollutants, may be an important part of coastal nutrient cycles, and may cause excess nutrient loading, thereby potentially degrading the coastal water quality. Here we report on Year-1 results from a cooperative (USGS-UF-LSU) project that is investigating the role of submarine groundwater discharge into Indian River Lagoon, Florida.


The Indian River Lagoon system (Fig. 1) extends over 250 km along the east-central coast of Florida and consists of three inter-connected lagoonal basins: Mosquito, Banana River, and Indian River lagoons. Exchange of lagoon water with the Atlantic Ocean is limited to four tidal inlets (Sebastian, Ft. Pierce, St. Lucie and Jupiter) that occur in the southern reaches of Indian River lagoon.

Submarine groundwater discharge may occur wherever an aquifer is hydraulically connected to the sea through permeable bottom sediments; decreases with distance from shore; is directly affected by groundwater withdrawals; and is a potential point source for contaminants.

Figure 2. Parameters of submarine ground water discharge.

The following processes control the salinity of lagoon water: precipitation, the exchange of water through these inlets, wind, tidal forcing, evaporation, surface runoff and potential submarine groundwater discharge. In this system, the intensity and duration of wind have the most pronounced affect on lagoon water levels.

The overall objective of this project was to determine the rate and potential ecological significance of submarine groundwater discharge to Indian River Lagoon (Fig. 2).

Site location map.

Figure 3. Site location map for upper Indian River Lagoon, Florida.

The study area during the first year of the project included the northern most 10 km of the Indian River Lagoon (˜48 km2). Of the 28 sampling stations, 22 were arranged in shore-perpendicular transects; the remaining six stations were distributed within the lagoon center (Fig. 3).

At each station, lagoon and interstitial water samples were collected, and groundwater seepage rates were measured using conventional seepage meters. Interstitial water samples were obtained from four stations using custom-built multi-samplers. Six groundwater samples were collected from wells surrounding the lagoon.

Two additional samples were collected from tributaries to the lagoon including Turnbull Creek and Haulover Canal. Sampling of the seepage stations, groundwater wells, and tributaries occurred in May 1999, to coincide with the end of the normal dry season, and in August 1999, during the normal rainy season. A third trip in December 1999 was used only to sample interstitial water.

Hydrogeology

Hydrostratigraphy; click for enlargement.

Figure 4. Hydrostratigraphy of northeastern Florida. [larger version]

The hydrogeology along the northeastern coast of Florida can be broadly divided into two aquifer systems – the Surficial and the Floridan aquifer system (Fig. 4). Sand, silt and clays of the Intermediate confining unit, which constitutes most of the Hawthorn Formation, separates these two aquifer systems (Leve, 1970; Spechler, 1994).

The Surficial aquifer system consists of Miocene to Holocene interbedded sand, shell, silt, clay and dolomitic limestone strata. The Surficial aquifer system is mostly unconfined, although the hydrogeology can be very heterogeneous.

Four clastic, highly regional surficial aquifers border the Indian River Lagoon including Terrace, Atlantic Coastal Ridge, Ten-mile Ridge, and Inter-ridge aquifers. Terrace aquifer occurs on the barrier islands separating Indian River Lagoon from the Atlantic Ocean. The Atlantic Coastal Ridge aquifer occurs in the northwestern region of Indian River Lagoon. This aquifer is composed of the Pleistocene Anastasia Formation, and provides most of the water supply for towns on the western edge of the northern Indian River Lagoon (Mims and Titusville).

The Floridan aquifer system can be further divided into two water-bearing aquifers (Upper and Lower Floridan), separated by less permeable semi-confining units. The Upper Floridan aquifer in the study area corresponds to the Ocala Limestone and in some parts, the Avon Park Formation (Fig. 4). The Ocala Limestone is characterized by high permeabilities that can be enhanced along bedding planes, fractures and conduits.

Hydrograph

Figure 5. Hydrograph of a Titusville well (adapted from St. John's Water Management District, 2000) [larger version]

Significant variations in ground-water levels occur seasonally (Fig. 5). Superimposed on such seasonal variations is a long-term decrease in the potentiometric surface that is largely attributed to increased groundwater withdrawals (Fig. 6). Nonetheless, recent potentiometric surface maps of the Upper Floridan aquifer indicate elevations that are above sea level for the entire length of Indian River Lagoon.

Map, see caption.

Figure 6. Potentiometric change map [larger version]

Such potentiometric surface elevations increase from north to south, where the Hawthorn Formation increases in thickness. The elevated potentiometric surface of the Upper Floridan, combined with the general lack of a confining unit in the vicinity of the study area makes much of upper Indian River Lagoon a potential zone of submarine groundwater discharge.

Geochemistry

To derive estimates of ground-water seepage into Indian River Lagoon, the following suite of tracers, chemical constituents and sampling devices were measured or utilized: nutrients, Cl-, conductivity, pH, temperature, dissolved oxygen, 87Sr/86Sr, d18O, 223,224,226Ra, 222Rn, seep meters, multi-samplers, and benthic flux chambers (Martin et al., 2000). Seepage rates were spatially and temporally heterogeneous, yet similar to rates previously measured in Indian River Lagoon using identical techniques. The seepage rates ranged from 3 - 100 ml m-2 min-1 during May (dry season) to 22 - 144 ml m-2 min-1 during August (rainy season). The average value for all meters increased from 40 to 63 ml m-2 min-1 from the dry to the rainy season, implying that there may be a connection between rainfall and increased seepage rates. The heterogeneous nature of these rates is likely caused by fluctuations in sediment permeabilities and other geologic characteristics.

Radon-222 and Ra isotopes have previously provided regionally integrated estimates of seepage flux in varied coastal environments (Cable et al., 1996; Moore, 1999; Swarzenski et al.). Benthic fluxes of Ra to the Indian River Lagoon were calculated using three independent methods that rely on the activities of short-lived Ra isotopes

  1. lagoon budget,
  2. benthic flux chambers and
  3. pore-water modeling.

The first two methods yield direct measurements of flux across the sediment/water interface, whereas the third technique generates an indirect flux estimate on the basis of pore-water Ra profiles. Calculations of the benthic flux of Ra range up to almost 500 dpm m-2 day-1. Using 226Ra pore-water activities, a maximum upward subsurface water flow of about 5 - 17 cm day-1 is required to sustain these fluxes. These values are similar to the values measured directly with the seepage meters.

By using 222Rn and 226Ra as mass balance tracers of seepage flux to the northern Indian River Lagoon, it is possible to obtain measurements of seepage that are independent of the short-lived Ra isotopes. Assumptions required for this mass balance approach are that negligible effects were observed from surface water exchange to the lagoon, tides, and diffusion from the sediments. Analogous to the short-lived Ra isotopes, seepage fluxes measured on the basis of excess 226Ra activities are similar in magnitude to those estimated using seepage meters.

Each submarine groundwater discharge technique has individual strengths and weaknesses. Seepage meters provide a direct measurement of localized flow. They can also easily provide 'clean' seep water samples. However, seep meters may be susceptible to possible artifacts caused by interaction of tides and waves, although such limitations have not been thoroughly tested. The radioisotopes are less difficult to sample in the field than using seepage meters, but their measurement requires sophisticated laboratory equipment that is not widely available. One important characteristic of the radioisotope techniques is that they provide an integrated value of seepage rates across the entire lagoon. They are thus complementary to the seepage meter technique.

Chloride concentrations indicate that only a minor component (1-5%) of seep water originates from meteoric ground water. This implies that 95–99% of the interstitial water has to be recycled lagoon seawater. The isotopic concentration of strontium (87Sr/86Sr) was nearly identical in the seep water and lagoon water, yet was measurably lower than that in modern seawater. The 87Sr/86Sr ratios were also systematically lower during the rainy season, reflecting the greater influx of seep water into lagoon water and short groundwater residence times. Nutrient concentrations were 3–5 times elevated in the seep water over the lagoon water, and suggest that sediment/water interface exchange processes, such as submarine groundwater discharge, are critical components of coastal nutrient budgets (Johannes, 1980; Krest et al., 2000).

References

Cable, J.E., Bugna, G.C., Burnett, W.C., Chanton, J.P., 1996. Application of 222Rn and CH4 for assessment of ground water discharge to the coastal ocean. Limnol. Oceanogr. 41, 1347-1353.

Johannes, R.E., 1980. The ecological significance of the submarine discharge of ground water. Mar. Ecol. Prog. Ser. 3, 365-373.

Krest J.M., Moore W.S., Gardner L.R., Morris J.T., 2000. Marsh nutrient export supplied by ground water discharge: Evidence from radium measurements. Global Biogeochemical Cycles 14, 167-176.

Leve, G.W., 1970. The Floridan aquifer in Northeast Florida. Ground Water 6, 19-29.

Martin J.B., Cable, J.E., Swarzenski, P.W., 2000. Quantification of ground water discharge and nutrient loading to the Indian River Lagoon. St. Johns River Water Management District Report pp. 154.

Moore, W.S., 1999. The subterranean estuary: a reaction zone of ground water and seawater. Mar. Chem. 65, 111-125.

St. John's River Water Management District, August 2000. Hydrologic Conditions Report, pp. 71.

Spechler, R.M., 1994. Saltwater intrusion and the quality of water in the Floridan Aquifer system, northeastern Florida. U.S. Geol. Surv. Water Resources Invest. Report 92-4174, p 76.

Peter Swarzenski, Jonathan Martin, Jaye Cable, Rita Bowker, 2000, Quantifying Submarine Groundwater Discharge to Indian River Lagoon, Florida: U.S. Geological Survey Open-File Report 00-492. [Download this report as a PDF]


Map showing location of Indian River Lagoon.

See also...

Cable, J. E., Martin, J. B., Swarzenski, P. W., Lindenberg, M. K. and Steward, J. (2004), Advection Within Shallow Pore Waters of a Coastal Lagoon, Florida. Ground Water, v. 42 i. 7, p. 10111020, doi:10.1111/j.1745-6584.2004.tb02640.x

Abstract

Ground water sources can be a significant portion of a local water budget in estuarine environments, particularly in areas with high recharge rates, transmissive aquifers, and permeable marine sediments. However, field measurements of ground water discharge are often incongruent with ground water flow modeling results, leaving many scientists unsure which estimates are accurate. In this study, we find that both measurements and model results are reasonable. The difference between estimates apparently results from the sources of water being measured and not the techniques themselves. In two locations in the Indian River Lagoon estuarine system, we found seepage meter rates similar to rates calculated from the geochemical tracers 222Rn and 226Ra. Ground water discharge rates ranged from 4 to 9 cm/d using seepage meters and 3 to 20 cm/d using 222Rn and 226Ra. In contrast, in comparisons to other studies where finite element ground water flow modeling was used, much lower ground water discharge rates of ˜0.05 to 0.15 cm/d were estimated. These low rates probably represent discharge of meteoric ground water from land-recharged aquifers, while the much higher rates measured with seepage meters, 222Rn, and 226Ra likely include an additional source of surface waters that regularly flush shallow (< 1 m depth) sediments. This resultant total flow of mixed land-recharged water and recirculated surface waters contributes to the total biogeochemical loading in this shallow estuarine environment.

Download or view the PDF here: http://info.ngwa.org/gwol/pdf/042979918.pdf

 

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