Understanding sources of streamflow and nutrient concentrations are fundamental for the assessment of pollutant loadings that can lead to water quality impairments. The objective of this study was to evaluate the discharge of three main tributaries, draining different land uses with karst features, as well as their combined influences on total nitrogen (TN) and total phosphorus (TP) levels in reservoir-like embayment (R-E) on a stream entering Lake Marion, South Carolina. From 2007-2009, hydrology, TN, and TP data were collected from the 1,555-ha Chapel Branch Creek (CBC) watershed. In general, monthly streamflow in all tributaries was found to be −10% of rainfall, and as little as 0.1% in the smallest tributary. The third tributary flowed into a cave system and discharged via a cave spring (CS) into the embayment while gaining a sustained groundwater flow from a second cave (GW) system. The CS flow was substantially larger than the flow measured in the other tributaries. The small amount of rainfall that became surface flow and the large flow at the cave spring indicated a significant water loss from the surface watershed to subsurface flow or a groundwater source area substantially larger than the surface watershed. Nutrient concentrations in flows from tributaries draining various land uses were not significantly different (α ¼ 0.05) for most of the locations. A simple water balance was developed to estimate the RE outflow to Lake Marion using measured discharges from three tributaries, change in storage computed using a bathymetric survey, daily lake level changes, rainfall, and computed evaporation. Mean monthly TN and TP concentrations in the embayment were substantially lower than the observed means from the two tributary outlets and the CS into the embayment, indicating a loss in the embayment. The second cave system at CS, representing an unknown subsurface drainage area, was the source of nearly 50% of TP loading, over 50% of flow, and over 70% of TN loading to CBC. These results may have implications in water quality management of the CBC watershed.

Karst watersheds possess both diffuse and conduit flow and varying degrees of connectivity between surface and groundwater over spatial scales that result in complex hydrology and contaminant transport processes. The flow regime and surfacegroundwater connection must be properly identified and characterized to improve management in karst watersheds with impaired water bodies, such as the Chapel Branch Creek (CBC), South Carolina watershed, which has a long-term sampling station presently listed on an EPA 303(d) list for phosphorous, pH, and nitrogen. Water from the carbonate limestone aquifer of the Santee Cave system and spring seeps in the CBC watershed were monitored to characterize dominant flow type and surface-groundwater connection by measuring dissolved calcium and magnesium, total suspended solids, volatile suspended solids, alkalinity, pH, specific conductance, and stable isotopes (d 18 O, d 2 H). These measurements indicated that the conduit flow to Santee Cave spring was recharged predominantly from diffuse flow, with a slow response of surface water infiltration to the conduit. Qualitative dye traces and stage elevation at Santee Cave spring and the adjacent Lake Marion (equal to the elevation of the flooded portion of CBC) also indicated a relation between fluctuating base level of the CBC reservoir-like embayment and elevation of the Santee Limestone karst aquifer at the spring. Methods described herein to characterize the flow type and surface-groundwater connection in the Santee Cave system can be applied not only to watershed management in the Chapel Branch Creek watershed, but also to the greater region where this carbonate limestone aquifer exists.

SWAT is a GIS-based basin-scale model widely used for the characterization of hydrology and water quality of large, complex watersheds; however, SWAT has not been fully tested in watersheds with karst geomorphology and downstream reservoir-like embayment. In this study, SWAT was applied to test its ability to predict monthly streamflow dynamics for a 1,555 ha karst watershed, Chapel Branch Creek, which drains to a large embayment and is comprised of highly diverse land uses. SWAT was able to accurately simulate the monthly streamflow at a cave spring (CS) outlet draining mostly agricultural and forested lands and a golf course plus an unknown groundwater discharging area, only after adding known monthly subsurface inputs as a point source at that location. Monthly streamflows at two other locations, both with multiple land uses, were overpredicted when lower lake levels were prevalent as a result of surface water flow to groundwater (losing streams). The model underpredicted the flows during rising lake levels, likely due to high conductivity and also a deep percolation coefficient representing flow lost to shallow and deep groundwater. At the main watershed outlet, a wide section performing as a reservoir embayment (R-E), the model was able to more accurately simulate the measured mean monthly outflows. The RE storage was estimated by using a daily water balance approach with upstream inflows, rainfall, and PET as inputs and using parameters obtained by bathymetric survey, LiDAR, and downstream lake level data. Results demonstrated the substantial influence of the karst features in the water balance, with conduit and diffuse flow as an explanation for the missing upstream flows appearing via subsurface conveyance to the downstream cave spring, thus providing a more accurate simulation at the embayment outlet. Results also highlighted the influences of downstream lake levels and karst voids/conduits on the watershed hydrologic balance. Simulation performance of hydrology could be improved with more accurate DEMs obtained from LiDAR for karst feature identification and related modification of SWAT parameters. This SWAT modeling effort may have implications on nutrient and sediment loading estimates for TMDL development and implementation in karst watersheds with large downstream embayments that have significant changes in water level due to adjoining lakes.

The SWAT model was used to predict total phosphorus (TP) loadings for a 1555-ha karst watershed-Chapel Branch Creek (CBC)-which drains to a lake via a reservoir-like embayment (R-E). The model was first tested for monthly streamflow predictions from tributaries draining three potential source areas as well as the downstream R-E, followed by TP loadings using data collected March 2007-October 2009. Source areas included 1) a golf course that received applied wastewater, 2) urban areas, highway, and some agricultural lands, and 3) a cave spring draining a second golf course along with agricultural and forested areas, including a substantial contribution of subsurface water via karst connectivity. SWAT predictions of mean monthly TP loadings at the first two source outlets were deemed reasonable. However, the predictions at the cave spring outlet were somewhat poorer, likely due to diffuse variable groundwater flow from an unknown drainage area larger than the actual surface watershed, for which monthly subsurface flow was represented as a point source during simulations. Further testing of the SWAT model to predict monthly TP loadings at the R-E, modeled as a completely mixed system, resulted in their over-predictions most of the months, except when high lake water levels occurred. The mean monthly and annual flows were calibrated to acceptable limits with the exception of flow over-prediction when lake levels were low and surface water from tributaries disappeared into karst connections. The discrepancy in TP load predictions was attributed primarily to the use of limited monthly TP data collected during baseflow in the embayment. However, for the 22-month period, over-prediction of mean monthly TP load (34.6 kg/mo) by 13% compared to measured load (30.6 kg/mo) in the embayment was deemed acceptable. Simulated results showed a 42% reduction in TP load due to settling in the embayment.

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