Trace Organic Compounds in the Drainfield of Onsite Wastewater Treatment Systems.

Domestic wastewater contains a complex mixture of trace organic compounds (TOrC) that originate from various intentional usages in the kitchen, sinks, toilets, and washers in the households. As TOrC are not completely removed in onsite wastewater treatment systems (OWTS), they can potentially contaminate groundwater. Our objective was to understand the occurrence, behavior, and leaching of TOrC in drainfield of a conventional OWTS. Our specific objectives were to: 1) estimate the total amount of the TOrC transported with leachate upon received septic tank effluent (STE) and 2) assess the mitigation of the TOrC by OWTS. Three small mounds (1.5 m long and 0.9 m wide with 1:1 side slope ratio; hereafter called micro-mounds) were constructed. Each mound was packed with 7.5 cm of sand and gravel mixture, followed by 30 cm of natural soil (bulk density; BD: 1.7 g/cm³), and 30 cm of sand (BD: 1.6 g/cm³). A drip line with three emitters (dispersing 9 liters of effluent per day) was placed on the top of sand layer, then a drip line was covered with 15 cm of sand, and St. Augustine grass was planted to mimic a residential OWTS. Each micro-mound was instrumented with 10 multi-probe sensors at various depths and locations to measure daily moisture, electrical conductivity, and temperature. Leachate was collected from micro-mounds each day in the beginning of the study (n =30) and then a weekly flow-weighted sample (n=75) was obtained. Collected samples were analyzed for selected TOrC by solid-phase extraction and liquid chromatography-tandem mass spectrometry (LC-MS/MS). St. Augustine grass samples were collected monthly to investigate plant uptake of TOrC. Soil cores from different depths were obtained at the end of experiment to determine accumulation and distribution of TOrC in various depths of soil layers. This presentation will discuss our ongoing laboratory and field scale research to address TOrC dynamics in OWTS.