Drivers of Sediment Accumulation and Nutrient Burial in Coastal Stormwater Detention Ponds, South Carolina, USA

Abstract

Stormwater detention ponds are widely utilized as control structures to manage runoff during storm events. These ponds also represent biogeochemical hotspots, where carbon (C) and nutrients can be processed and buried in sediments. This study quantified C and nutrient [nitrogen (N) and phosphorus (P)] sources and burial rates in 14 stormwater detention ponds representative of typical residential development in coastal South Carolina. Bulk sediment accumulation was directly correlated with catchment impervious surface coverage (R2 = 0.90) with sediment accumulation rates ranging from 0.06 to 0.50 cm y−1. These rates of sediment accumulation and consequent pond volume loss were lower than anticipated based on maintenance guidelines provided by the State. N-alkanes were used as biomarkers of sediment source; the derived terrestrial aquatic ratio (TARHC) index was strongly correlated with sediment accumulation rate (R2 = 0.71) which, in conjunction with high C/N ratios (16–33), suggests that terrestrial biomass drives this sediment accumulation, with relatively minimal contributions from algal derived material. This is counter to expectations that were based on the high algal productivity generally observed in stormwater ponds and previous studies of natural lakes. Sediment C and nutrient concentrations were consistent among ponds, such that differences in burial rates were a simple function of bulk sediment accumulation rate. These burial rates (C: 8.7–161 g m−2 y−1, N: 0.65–6.4 g m−2 y−1, P: 0.238–4.13 g m−2 y−1) were similar to those observed in natural lake systems, but lower than those observed in reservoirs or impoundments. Though individual ponds were small in area (930–41,000 m2), they are regionally abundant and, when mean burial rates are extrapolated to the regional scale (≈ 21,000 ponds), ultimately sequester 2.0 × 109 g C y−1, 9.5 × 107 g N y−1, and 3.7 × 107 g P y−1 in the coastal region of South Carolina alone. Stormwater ponds represent a relatively new but increasingly significant feature of the coastal landscape and, thus, are a key component in understanding how urbanization alters the transport and transformations of C and nutrients between terrestrial uplands and downstream receiving waters.