Protecting groundwater resources at biosolids recycling sites.

In developing the national biosolids recycling rule (Title 40 of the Code of Federal Regulation Part 503 or Part 503), the USEPA conducted deterministic risk assessments whose results indicated that the probability of groundwater impairment associated with biosolids recycling was insignificant. Unfortunately, the computational capabilities available for performing risk assessments of pollutant fate and transport at that time were limited. Using recent advances in USEPA risk assessment methodology, the present study evaluates whether the current national biosolids pollutant limits remain protective of groundwater quality. To take advantage of new risk assessment approaches, a computer-based groundwater risk characterization screening tool (RCST) was developed using USEPA's Multimedia, Multi-pathway, Multi-receptor Exposure and Risk Assessment program. The RCST, which generates a noncarcinogenic human health risk estimate (i.e., hazard quotient [HQ] value), has the ability to conduct screening-level risk characterizations. The regulated heavy metals modeled in this study were As, Cd, Ni, Se, and Zn. Results from RCST application to biosolids recycling sites located in Yakima County, Washington, indicated that biosolids could be recycled at rates as high as 90 Mg ha, with no negative human health effects associated with groundwater consumption. Only under unrealistically high biosolids land application rates were public health risks characterized as significant (HQ ≥ 1.0). For example, by increasing the biosolids application rate and pollutant concentrations to 900 Mg ha and 10 times the regulatory limit, respectively, the HQ values varied from 1.4 (Zn) to 324.0 (Se). Since promulgation of Part 503, no verifiable cases of groundwater contamination by regulated biosolids pollutants have been reported.

Use of biosolids to enhance rangeland forage quality.

Biosolids land application was demonstrated to be a potentially cost-effective means for restoring forage productivity and enhancing soil-moisture-holding capacity on disturbed rangelands. By land-applying aerobically digested, anaerobically digested, composted, and lime-stabilized biosolids on rangeland test plots at rates of up to 20 times (20X) the estimated nitrogen-based agronomic rate, forage yields were found to increase from 132.8 kg/ha (118.2 lb/ac) (control plots) to 1182.3 kg/ha (1052.8 lb/ac). Despite the environmental benefits associated with increased forage yield (e.g., reduced soil erosion, improved drainage, and enhanced terrestrial carbon sequestration), the type of forage generated both before and after biosolids land application was found to be dominated by invasive weeds, all of which were characterized as having fair to poor nutritional value. Opportunistic and shallow rooting invasive weeds not only have marginal nutritional value, they also limit the establishment of native perennial grasses and thus biodiversity. Many of the identified invasive species (e.g., Cheatgrass) mature early, a characteristic that significantly increases the fuel loads that support the increased frequency and extent of western wildfires.

Risk characterization, assessment, and management of organic pollutants in beneficially used residual products.

A wide array of organic chemicals occur in biosolids and other residuals recycled to land. The extent of our knowledge about the chemicals and the impact on recycling programs varies from high to very low. Two significant challenges in regulating these materials are to accurately determine the concentrations of the organic compounds in residuals and to appropriately estimate the risk that the chemicals present from land application or public distribution. This paper examines both challenges and offers strategies for assessing the risks related to the occurrence of organic compounds in residuals used as soil amendments. Important attributes that must be understood to appropriately characterize and manage the potential risks for organic chemicals in biosolids include toxicity and dose response, transport potential, chemical structure and environmental stability, analytical capability in the matrix of interest, concentrations and persistence in waste streams, plant uptake, availability from surface application versus incorporation, solubility factors, and environmental fate. This information is complete for only a few chemicals. Questions persist about the far greater number of chemicals for which toxicity and environmental behavior are less well understood. This paper provides a synopsis of analytical issues, risk assessment methodologies, and risk management screening alternatives for organic constituents in biosolids. Examples from experience in Wisconsin are emphasized but can be extrapolated for broader application.

Long-term biosolids application effects on metal concentrations in soil and bermudagrass forage.

The long-term application of biosolids that periodically contained elevated metal concentrations has raised questions about potential effects on animal health. To address these concerns, we determined metal concentrations (As, Cd, Cu, Pb, Hg, Mo, Ni, Se, and Zn) in both soil and bermudagrass [Cynodon dactylon (L.) Pers.] forage from 10 fields in the following categories of biosolids application: six or more years (>6YR), less than six years (<6YR), and no applications (NS). Soil metal concentrations in all groups were similar to values reported for mineral soils in Georgia, and well below USEPA cumulative limits. Average metal concentrations in the forage were below the maximum tolerable level (MTL) for beef cattle, although two biosolids-amended fields in the >6YR group produced forage that was at or near the MTL for Cd and Mo, and one field in the <6YR group produced forage above the MTL for Cd. The Cu to Mo ratios in forage decreased with increasing time of sludge application, with the average in the >6YR group at a proposed 5:1 Cu to Mo ratio limit to protect ruminant health. Sulfur concentrations in the forage from all three groups was near the MTL of 4 g kg(-1). The study indicated that toxic levels of metals have not accumulated in the soils due to long-term biosolids application. Overall forage quality from the biosolids-amended fields was similar to that of commercially fertilized fields; however, due to the relatively high S and potential for a low Cu to Mo ratio, Cu supplements should be used to ensure ruminant health.

Groundwater quality protection at biosolids land application sites.

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 (R(f)d) 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).