The spatial variability of nitrogen and phosphorus concentration in a sand aquifer influenced by onsite sewage treatment and disposal systems: a case study on St. George Island, Florida
Introduction
Groundwater discharge provides a significant amount of nutrients and contaminants into some coastal zones (Vailela et al., 1978, Valiela and Teal, 1979, Capone and Bautista, 1985, Lapointe and O'Connell, 1989, Capone and Slater, 1990, Lapointe et al., 1990, Valiela et al., 1990). In areas with a shallow freshwater system, groundwater may easily be contaminated from onsite sewage treatment and disposal systems (OSTDS), which are typically installed less than 1 m above the water table and may be flooded during heavy rains. The transport of non-point source pollution from OSTDS to shallow groundwater and ultimate discharge to surface waters could be an important source of contamination to the marine environment, especially in areas of restricted circulation such as an estuary or small embayment. While this pathway is becoming better known, it is still largely ignored by scientists and coastal managers alike. When evaluating impacts from OSTDS on coastal waters, it is important to consider the natural flux of nutrients in order to implement sound management strategies.
St. George Island, like many barrier islands, forms the outer perimeter of an estuary (Apalachicola Bay) and is critical to the bay's productivity because its orientation determines the salinity distribution as well as other water quality features of the bay. Apalachicola Bay is one of the most economically important estuarine systems in Florida due to oyster and shrimp harvesting. The island, which is located within the Apalachicola National Estuarine Reserve, is developing at a rapid pace. This has resulted in a relatively high density of OSTDS on the island. Growth in these and surrounding communities is of major concern with regard to the health of the estuary, which is likely sensitive to slight physical, chemical, and biological perturbations. Although barrier islands play a critical role in this balance, very little is known about the groundwater dynamics and potential impacts of contaminated groundwaters on the surrounding waters.
In order to prevent the possible deterioration of Apalachicola Bay and other estuarine systems, including economic zones (oyster beds and areas of dense shrimp populations), contaminants of any type must be monitored closely. Although the Apalachicola River (the largest river in Florida) clearly provides the majority of the nutrients to the bay, those supplied via groundwater transport from St. George Island may be locally important. Without knowledge of the groundwater contribution, interpretation and management decisions concerning the treatment of sewage may be faulty and lead to future environmental threats. Thus, monitoring of OSTDS in an area of increasing development and density is necessary to help guide future wastewater treatment decisions.
Virtually all the homes and businesses on St. George Island have an onsite wastewater treatment and disposal system with a drainfield less than 1 m above the shallow water table. We showed earlier using artificial tracers (Corbett et al., 2000) that average groundwater velocities are as high as 0.4 m day−1 and presumably are even greater during large rain events. The weakly dispersive nature of the island's aquifer and the fairly rapid transport rates may make current minimum distance-to-surface water (∼23 m) regulations for permitting wastewater systems inadequate for protecting local surface water quality. We collected wastewater as it exited the OSTDS and monitored the groundwater nutrient concentrations in the surrounding groundwaters at selected sites on St. George Island (Fig. 1). Multi-level samplers (MLS) and 5 cm PVC monitoring wells were placed downgradient from three wastewater systems at different locations on the island. These sites included two types of onsite sewage treatment and disposal systems, aerobic and anaerobic (septic) treatment. Compositional differences in the effluent discharged and possible groundwater contaminants from both types of systems have been evaluated. The wells were monitored over the course of the study for nutrient concentrations along the flow path toward surface waters. Two sampling locations on undeveloped Little St. George Island (Fig. 1), a small barrier island adjacent to the main island was established to assess reasonable background levels.
Section snippets
St. George Island
St. George Island, a barrier island in the Panhandle of Florida, is approximately 48 km long and averages less than 0.5 km in width. Dr. Julian G. Bruce State Park occupies the east end of the island. The climate in the region is mild with a mean annual temperature of approximately 20°C (Livingston, 1984). The mean annual rainfall over the area, recorded over the last 42 years by the NOAA weather station in Apalachicola, is approximately 140 cm, 40% occurring during the late summer months. The
Materials and methods
Monthly measurements of hydrographic and chemical variables were made from September 1997 to May 1999 at the SP site and from April 1998 to March 1999 at the BL and JA field sites. Water samples were collected in 125 ml acid washed polypropylene bottles. Samples for nutrient analyses were filtered through GF/F filters (0.7 μm) in the field and were stored on ice in the dark until analysis. Nutrients were analyzed within 24 h of sample collection. Nitrate (NO3−) and total dissolved nitrogen (TN)
Nutrient dynamics
St. George Island's drinking water is obtained from three deep wells (>150 m) on the mainland, which is then pumped over to a large storage tank (570 m3) on the island. Interestingly, the silicate concentration in the tap water averaged 11.5 mg Si l−1, ranging from 8.1 to 14.3 mg Si l−1 throughout the study period at all the experimental sites, which is four times higher than the concentration in the unimpacted surficial aquifer. Groundwater collected from Little St. George Island, indicative
Summary
Nutrient concentrations monitored downgradient from wastewater disposal systems show significant attenuation before discharge into surface waters. Silicate was used as a natural conservative tracer, providing insight to the extent of the wastewater plume and dilution of other nutrients. Total nitrogen, ammonia, and total phosphate were all attenuated relative to silicate, indicating non-conservative removal of these nutrients in the subsurface. Estimates of the total nitrogen flux into
Acknowledgements
We would like to thank Bo Tapnell, Scott Powell, Greg Smith, Trevor Popp, and Roger Wong for their assistance with fieldwork. We would also like to thank the National Park Service on St. George Island, Jay Abbott, and B.L. Cosey for allowing the use of their property to install wells for this research. Lee Edmiston and Chip Bailey at the Apalacicola National Estuarine Research Reserve provided the use of their equipment and boats and assistance in the field when ever necessary.
The NOAA's
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