The effects of varied hydraulic and nutrient loading rates on water quality and hydrologic distributions in a natural forested treatment wetland

Hydrology and water quality distributions in a Louisiana forested waste-water treatment wetland were studied under four different hydraulic loading rates (HLR). Pond discharge, surface-water elevations, and fluorescent dye travel times were recorded to assess surface-water hydrology, and water samples were collected for nitrate, ammonium, phosphate, and suspended solids analyses. Wetted surface area increased with pond discharge rate, and 58 to 66 percent of surface-water flow was concentrated in shallow channels covering only 10 to 12 percent of the total study area. Water residence times were much longer (0.9 to 1.1 days) than minimum dye travel times (2 to 3 hours) through the 4-hectare study area. Relative to study area influent concentrations, study area outflow concentrations of nitrate and total and organic suspended solids were lower, ammonium was higher, and phosphate was generally unchanged. However, there was an increase in concentrations of nitrate, ammonium, and phosphate within 50 m of the study area inflow location. Ammonium and phosphate did decrease from these peak concentrations. Net nitrate production was observed within 50 m of the pond outfall and was probably due to nitrification. Net nitrate removal was observed beyond this distance and ranged up to 0.10 . g . m(-2) . d(-1) probably due primarily to denitrification. In general, nitrate removal rates increased linearly with changes in nitrate loading rates. Results show that nutrient distributions are linked to hydrology. Higher pond discharge rates created more treatment surface area, and higher constituent loading rates produced higher removal rates. Therefore, discharge rates could be manipulated, and physical control structures could be installed to increase wetted surface area and increase removal efficiency within the wetland. Higher loading rates could then be processed without requiring significant increases in treatment area.