Groundwater quality protection at biosolids land application sites

Abstract

Using the United States (US) Environmental Protection Agency's (EPA) Multimedia, Multi-pathway, Multi-receptor Exposure and Risk Assessment (3MRA) technology, a computer-based biosolids groundwater risk characterization screening tool (RCST) was developed. The RCST, which generates a non-carcinogenic human health risk estimate (i.e., hazard quotient or HQ value), has the ability to conduct screening-level risk-based characterization of potential human risks associated with pollutants released from biosolids land application sites. The HQ is a human health indicator that is equal to the ratio of the pollutant dose (mass of pollutant per unit body weight per time) to the specific pollutant reference dose (Rfd) which, in turn, is a human health benchmark defined by the EPA as a scientific estimate of the daily exposure level. A HQ value equal to or greater than one (1) suggests that the resulting conditions pose an unacceptable risk to human health.The focus of the current study was to evaluate whether the present regulatory limits established for biosolids pollutants (e.g., heavy metals) were sufficiently protective of human health associated with potential groundwater consumption using a new EPA risk assessment tool. Application of the RCST to two biosolids land application sites located near Columbus, Georgia predicted that, when the depth to groundwater was maintained at a distance of at least 2 m, regulated pollutant concentrations as large as ten (10) times the current regulatory limit (i.e., Title 40 of the US Code of Federal Regulations Part 503 – Ceiling Concentration Limit) could be safely land applied at rates as high as ninety (90) Megagrams per hectare (Mg ha−1) with no apparent non-carcinogenic human health effects associated with groundwater consumption. At these pollutant concentrations, the HQ ranged from 1.79 × 10−9 for cadmium to 3.03 × 10−3 for selenium.Only under unrealistically high biosolids application rates were the public health risks associated with groundwater impairment characterized as significant (HQ ≥ 1.0). For example, when the biosolids application rate was increased to 450 Megagrams per hectare (Mg ha−1) and the pollutant concentrations were increased to ten times the 40 CFR Part 503 Ceiling Concentration Limit, a HQ value of 2.23 was estimated (selenium). Similarly, when the biosolids application rate was increased to 900 Mg ha−1 and the pollutant concentrations were increased to ten times the regulatory limit, the HQ ranged varied from 1.4 (for zinc) to 324.0 (for selenium).