A DIRECT MEASUREMENT TECHNIQUE TO DETECT OSDS CONTAMINANT LOADING TO AQUATIC SYSTEMS

It appears the water quality of the Indian River Lagoon System (IRL), a vital biological and economic resource located on the east coast of Florida, is declining and many experts feel the high number of OSDS (on-site detention systems) in the watershed may be contibuting greatly to this decline. In view of this, direct measurement studies behind individual residences with OSDS were employed to determine the impact of these systems on the IRL and St. Lucie Estuary--a tributary to the IRL. This approach avoids the errors and assumptons associated with modeling, relying instead on directly measured concentrations and groundwater seepage rates for use in loading estimates. This methodology, can be easily used to investigate individual OSDS sites and involves the use of PushPoint Samplers to accurately locate and track contaminated groundwater down gradient from the OSDS drainfield, 3/4 inch piezometers to establish groundwater levels and seepage meters to determine groundwater seepage rates. The PushPoint used was a 1/4 inch diameter tubular 316 stainless steel rod with interlaced slots which form a screened zone. The PushPoint was attached to a peristaltic pump for sampling and groundwater samples were obtained at various depths in transects in the soil and benthic sediment behind OSDS installations to track plume migration. The concentration data at the edge of the waterbody was multiplied by the average seepage rate to estimate loading rates.

Although 2005/2006 data from six to seven sites are presented, results to date indicate OSDS loading to the St. Lucie Estuary was occurring at two of these sites. One greater than 50 year old site with predominately coarse sandy soil exhibited extemely elevated concentrations of nitrate-nirogen 76 ft downgradient from the drainfield and was discharging to the St. Lucie Estuary. At other sites, nitrate-nitrogen, ammonia-nitrogen, soluble reactive phosphate and fecal coliform bacteria were found in elevated concentrations at distances 20 to 65 ft from the edge of the drainfield. Soil characterics, hydraulic gradents, rainfall, OSDS age and loading characteristics determined the plume migration distances at each site.