An Examination of a Dune Infiltration System's Impact on Coastal Hydrology and Bacteria Removal

Abstract

The Beaches Environmental Assessment and Coastal Health Act of 2000 (BEACH Act) requires states to monitor bacteria levels in recreational coastal waters. Increased levels of bacteria increase the potential for many illnesses to beach goers, so coastal towns are forced to post advisories or close beaches after many rainfall events, which potentially negatively impacts tourism. Stormwater outfalls, common in many coastal towns, discharge stormwater and associated bacteria and other pollutants into the ocean or estuaries.

The NC Department of Transportation and the Town of Kure Beach wanted to reduce the amount of stormwater discharging on Kure Beach's recreational beach. Two stormwater Dune Infiltration Systems were designed to divert a portion of the flow into the beach dunes. Sand filters have historically been successful in bacterial removal. The infiltration systems were constructed using commercially-available open-bottomed infiltration chambers. Due to limited land area, the systems were designed to infiltrate 1.3 cm storms, which comprise approximately 80% of the rainfall events at the site. The watersheds of both sites (L and M) were small (1.9 ha and 3.2 ha respectively) and of mixed urban and residential land use. Water table measurements indicated a tidal influence, but approximately 2 m of sand was available for infiltration in the vertical direction.

Data were collected from twenty-five storms during the months of March through October 2006 to determine the Dune Infiltration System's viability as a BMP. From those 25 storms, Site L's Dune Infiltration System captured total volume of 645 m3 of stormwater runoff. Site M's Dune Infiltration System capacity was exceeded during 20 percent of the storms, capturing a total stormwater runoff volume of 2313 m3 of the total 2412 m3. At both sites, the Dune Infiltration Systems significantly (p < 0.01) reduced runoff volume and peak flow discharging directly onto the beach. Routing the stormwater runoff through the dune's soil and into groundwater below did not cause noticeable fluctuations in the groundwater.

Bacteria concentrations in the stormwater runoff flowing into the Dune Infiltration System ranged from 181 CFU⁄100ml to 19400 CFU⁄100ml with a median of 8600 CFU⁄100ml for fecal coliform concentrations and from <10 CFU⁄100ml to >2005 CFU⁄100ml with a median of 1298 CFU⁄100ml for enterococcus. The groundwater bacteria concentrations were significantly (p<0.01) lower than the inflow, ranging from <1 CFU⁄100 ml to 214 CFU⁄100 ml with a median of 1.5 CFU⁄100ml for fecal coliform concentrations and from <10 CFU⁄100 ml to 2005 CFU⁄100 ml with a median of 10 CFU⁄100ml for enterococcus concentrations. Groundwater bacteria levels at Site L never exceeded North Carolina state's standard; whereas, 23 percent of groundwater samples from Site M did. The samples that exceeded the standards were towards the end of the study and associated with relatively large runoff volumes.

A laboratory experiment was performed on Kure Beach's soil to analyze the effect bacteria had on infiltration and vice verse. Soil columns were treated with bacteria- free stormwater or bacteria-spiked (Escherichia coli) stormwater. It was found at a 95% confidence level that the infiltration rate of the Escherichia coli stormwater treatment was statistical lower than the bacteria-free columns' infiltration rate. A correlation found was between the concentrations of bacteria found in the Escherichia coli stormwater treatment columns' effluent to the infiltration rate of these columns.

The Dune Infiltration System's viability as a stormwater BMP requires continued research however, initial results are promising. The Dune Infiltration Systems did reduce the amount and rate of stormwater directly discharging into the ocean, while maintaining groundwater hydrology. Specifically more research is needed to better understand the Dune Infiltration System's bacteria removal efficiency. As demonstrated in the laboratory study, sediment accumulation can cause diminished infiltration rates, but correspondingly decreasing the bacteria concentration in the groundwater. The reduction in infiltrations rates result in a decrease in the system's retention volume, potentially leading to more overflows. The relationship between infiltration rate to bacteria removal needs to be evaluated when designing the Dune Infiltration System and developing a maintenance schedule.