ASSESSING SURFACE AND GROUNDWATER NUTRIENT INPUTS TO A COASTAL LAKE, LONG LAKE, SOUTH CAROLINA

Long Lake (Myrtle Beach, SC), has historically been a clear, mineral bottom lake (fine-medium quartz sand with calcareous clay lenses) dominated by sparse macroflora. However, over the last decade, the lake has become dominated by the macroalga Chara sp. It was hypothesized that increased nutrient inputs associated with surrounding land use changes (urbanization) could have influenced the growth of Chara. This study (2014-2015) focused on quantifying nutrient inputs to the lake. We used surface water and groundwater sampling, geo-electrical surveys, and lake water quality surveys to document water quality, surface water, and groundwater fluxes to the lake. In addition, five cores (~ 7-15 m depth) collected by the SC Geological Survey were used for hydrogeological characterization. Specific conductivity, temperature, water table elevation, nutrient, and chloride data were collected during three dates in 2015. A nutrient-rich tributary northwest of the lake supplies nutrients to the lake, but preliminary data indicate that the major flux is transported by groundwater feeding the lake. These data also suggest that in some locations septic systems may contribute nitrogen to the lake. The lake water column was slightly alkaline (median pH 8.6) and oligotrophic (0.2-6.8, median = 1.3 mg/L chlorophyll a) with typically low DIN and phosphate concentrations (0.015-2.06 mg/L, median DIN = 0.03 mg/L, 0-0.013, median phosphate = 0.003 mg/L). In contrast, nutrient concentrations in groundwater directly underlying the lake were higher. We sampled groundwater at 6-8 inches below the lake bottom at 40 locations using temporary piezometers. In most locations, there was a carbonate and organic matter-rich flocculent layer coating the mineral sediments at the bottom of the lake. Shallow lake sediment porewaters frequently contained elevated dissolved nitrogen (0.7-24.85 mg/l, median TDN= 2.3 mg/l) relative to surface water, and the dominant form of lake porewater nitrogen was ammonium, presumably related to decomposition of organic matter. In addition, brackish groundwater (Cl>500 mg/l) existed at 35% of the sample sites underlying the lake. Future work will include stable isotope analyses to assist in determining the groundwater nitrate source and management recommendations to help reduce nutrient inputs to the lake.