Assessing Potential Impacts of a Wastewater Rapid Infiltration Basin System on Groundwater Quality: A Delaware Case Study

Abstract

Rapid infiltration basin systems (RIBS) are receiving increased interest for domestic wastewater disposal in rural areas. They rely on natural treatment processes to filter pollutants and use extremely high effluent loading rates, much greater than natural precipitation, applied to a small geographic area instead of disposal to surface water. Concerns exist today that adopting RIBS in areas with shallow groundwater and sandy soils may increase ground and surface water pollution. We conducted a field study of RIBS effects on N and P concentrations in soils and groundwater at Cape Henlopen State Park, Delaware, where a RIBS designed and operated following USEPA guidance has been used for >25 yr. Site and wastewater characteristics (water table of 8 m, Fe- and Al-oxide coatings on soils, organic-rich effluent) were favorable for denitrification and P sorption; however, we found high P saturation, reduced soil P sorption capacity, and significant total P accumulation at depths >1.5 m, factors that could lead to dissolved P leaching. Very low soil inorganic N levels suggest that wastewater N was converted rapidly to NO-N and leached from the RIBS. Extensive groundwater monitoring supported these concerns and showed rapid offsite transport of N and P at concentrations similar to the effluent. Results suggest that high hydraulic loads and preferential flow led to flow velocities that were too large, and contact times between effluent and soils that were too short, for effective N and P attenuation processes. These findings indicate the need for better site characterization and facility designs to reduce and monitor contaminant loss from RIBS in similar settings.

Development of phosphorus sorption capacity‐based environmental indices for tile‐drained systems

“…Likewise, our analysis of WSP as a function of SPSC was generally in keeping with recent work. In previous studies, negative SPSC values were associated with significant increases in soluble P release from soils (Andres & Sims, 2013;Chakraborty et al, 2012;Nair & Harris, 2014;Oladeji, O'Connor, Sartain, & Nair, 2007). Similarly, in our study, negative SPSC values were associated with a significant linear increase in WSP loss (R 2 = 0.52) (Figure 2a).…”

“…Similarly, in our study, negative SPSC values were associated with a significant linear increase in WSP loss (R 2 = 0.52) (Figure 2a). Sites with no recent history of P management (i.e., TPAC [ Table 2]) had positive SPSC values with minimum P release to soil solution, a result consistently observed in prior work (Andres & Sims, 2013;Nair & Harris, 2004, 2014Oladeji et al, 2007). Due to the rapid rate of change between negative SPSC and WSP compared with positive SPSC, zero SPSC has been suggested as an environmental threshold (Nair & Harris, 2004); our results support using a SPSC threshold value close or equal to zero, as no significant linear relationship existed between WSP values <0.01 and SPSC.…”

Evaluation of Legacy Phosphorus Storage and Release from Wetland Soils

“…A recent literature review of threshold PSR values by researchers in different parts of the world (Nair, 2014) indicated that most soils tend to have a threshold PSR in the 0.10 to 0.15 range. Using a threshold PSR of 0.15, Andres and Sims (2013) showed that the PSR–SPSC concept was effective in risk assessment of P loss from a groundwater field monitoring site in Delaware, USA. Based on a visual observation of the threshold PSR of 0.04, Bhadha et al (2010) noted that SPSC values of surface soils in both wetlands and uplands in their study were negative and EPC 0 values and pore water P concentrations were high, indicating that all the soils were P sources.…”

Soil Phosphorus Storage Capacity for Environmental Risk Assessment

“…Others have documented an association between negative SPSC and elevated solution P both in the field (pore water [31]) and laboratory (column leachates [19]). …”