NITROGEN AND CARBON AVAILABILITY FROM LAND APPLIED TIMBER INDUSTRY RESIDUALS: IMPLICATIONS FOR GROUNDWATER IMPACTS AND STATE POLICY

The North Carolina Division of Water Quality is conducting a study at a residuals land application site with elevated groundwater nitrate levels, located in the western Piedmont region of NC. Residuals from a wood processing facility are collected from the onsite waste water treatment plant, dewatered, and applied to adjacent agricultural fields. The project goal is to assess whether current state requirements for permits and monitoring are adequate to protect water quality at application sites, through field data collection and laboratory experiments.

Site regulatory documents from 2002-2011 indicate PAN levels in residuals varied from 920 to 20,932 mg/kg dry waste material, with no consistent temporal trends. As expected for timber wastes, organic nitrogen (TKN) accounted for over 95% of PAN. Between 2005-2009, annual PAN loading from residuals was below projected crop demand three times and significantly above once. Batch leaching experiments were conducted on residuals to simulate a first flush rainfall. Extracted soluble TKN was 1650.5 ± 0.5 mg/kg dry matter, NH₃ was 240.7 ± 6.2 mg/kg dry matter, combined NO₂ + NO₃ was below detection, and DOC was 32.9 ± 3.1 g/kg dry matter. The C:N ratio of soluble species was 15.7-19.8, somewhat higher than results for solid residuals (13.1-14.0) and generally higher than expected for wood wastes, possibly due to addition of microbial biomass during wastewater treatment. The bioavailable fraction of soluble carbon was 84 ± 2% , determined by analyzing DOC losses in inoculated extract. Oxitop™ respirometers were used to estimate the BOD5 of extracts, which ranged from 1600-2000 mg/L.

Residuals at this site are likely to produce high dissolved carbon and nitrogen loads in the first flush; research on municipal wastes suggests C:N ratios below 20 will prompt N mobilization, but may not apply to non-composted woody residuals with high carbon bioavailability. Unique and/or highly variable wastes complicate predictions of nitrogen dynamics, particularly if N mineralization rates differ from those used to determine PAN for municipal wastewater residuals. State regulations do not require documentation of additional nutrient inputs to residuals land application fields, further complicating efforts to predict conditions likely to cause groundwater impacts.