elevation of the groundwater table . Additional freshwater recharge is provided by the extensive network of landscape irrigation , primarily introduced from sources external to Kiawah Island . Overtime , the groundwater elevation gets dispersed horizontally throughout the subsurface aquifer and eventually discharges through the marsh platform or into the unlined stormwater pond network . Groundwater will discharge to reach an equilibrium with tidal fluctuations , which is generally just above mean sea level on Kiawah Island . Groundwater discharge through the marsh subsurface dilutes the salinity of seawater around the root structures of marsh vegetation . Generally , locations in the marsh where the introduction of groundwater is reduced or impeded are where there are losses of certain high marsh vegetation .
If those soils become saturated or cannot percolate at the same rate stormwater is introduced , it creates stormwater runoff that naturally flows from higher to lower elevations . For Kiawah Island ’ s suburban community , the drainage system includes a series of interconnected ponds , which collect water from nearby residential areas . The stormwater runoff captured by these ponds eventually flows out into the marsh area from several major outfalls and a handful of outlets located on the marsh side of the island . Four major outfalls release around 95 % of stormwater runoff on the island and are found in Inlet Cove , River Course , Bufflehead Pond , and Canvasback Pond . KICA regulates the outfall structures at each of these outlets and stormwater is only released when the tides are lower than the levels within the pond . This limits the outflow of excess water generally during low tide events .
Undeveloped maritime forest habitats allow the highest capacity for infiltrating rainfall into the groundwater system . As areas become more developed , the amount of impervious surface is increased . Impervious surfaces cause runoff rates to increase and facilitate the transport of contaminants downstream by decreasing the capacity of rainfall to infiltrate into the soil . This effectively increases the volume of stormwater runoff into the pond infrastructure and into marshes . The gradual increase in volumetric flow of stormwater into the marshes affects the physical and chemical attributes of tidal streams .
As mentioned previously , stormwater also impacts the water quality in the pond and estuarine system . Because it contains a marginal level of dissolved oxygen , the introduction of stormwater lowers the available dissolved oxygen in open water areas . This can result in hypoxic conditions in ponds which can lead to fish kills . Additionally , stormwater can transport accumulated pollutants that flow off impervious surfaces and soils upstream . These pollutants include polycyclic aromatic hydrocarbons , heavy metals , pharmaceuticals and personal care products , pesticides , fertilizers , flame retardants , and microplastics , among others . The introduction of these pollutants results in negative impacts on the survival of local terrestrial and aquatic life .
Climate and Tidal Changes
Tidal inundation is a major concern for marsh health , as tides and elevation influence the zonation of marsh plant communities . Climatic change poses a long-term threat to the survival of marsh vegetation and estuarine mollusks surrounding Kiawah Island . As sea levels continue to rise , tidal fluctuations are shifted to higher elevations . As a result of this shift , lower elevations of the marsh will become inundated for longer periods of time and create adverse conditions for marsh grasses , especially smooth cordgrass which often form monocultures in lower areas of tidal wetland areas in South Carolina . Changes in local climate patterns can also result in droughts becoming more common and rainfall events being less frequent but more intense . This results in decreased elevations of the subsurface aquifer , reduced percolation of freshwater into the ground , and increased stormwater runoff . These conditions altogether will gradually cause the subsurface freshwater aquifer to rise and become more saline .
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