COMPARISON OF SOIL PHOSPHORUS ACCUMULATION IN WASTEWATER-IRRIGATED FORESTS AND CROPLAND

Open Access
- Author:
- Heyler, Timothy W
- Graduate Program:
- Environmental Pollution Control
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- April 05, 2011
- Committee Members:
- Herschel Adams Elliott, Thesis Advisor/Co-Advisor
Herschel Adams Elliott, Thesis Advisor/Co-Advisor - Keywords:
- forest
wastewater irrigation
soil phosphorus - Abstract:
- Land treatment of municipal wastewater effluent avoids direct nutrient discharges to surface waters, but the transport and fate of these nutrients in soil systems needs to be understood. The Pennsylvania State University has applied secondary treated wastewater effluent to forests and agricultural cropland from 1963 – 1976, and continuously since 1983 at the Living Filter (LF) site. The objective of this study was to characterize soil phosphorus (P) accumulation for contiguous wastewater-irrigated forested and grassed areas of the LF. An adjacent unirrigated area served as control. Soil cores were collected from the Hagerstown soil, and divided into five layers by depth (0 – 15, 15 – 30, 30 – 45, 45 – 60, and 60 – 75 cm). The samples were analyzed for Mehlich-3 extractable P, aluminum (M3Al), iron (M3Fe), copper, calcium, as well as pH, total P (TP), and percent organic matter content. A TP mass balance was performed for the crop field and forest soils for the top 75 cm of soil. Effluent applied P was estimated using historical data and average annual TP loading rates. Biomass removal was subtracted from applied P to determine the net P applied with wastewater irrigation. Control site soil data were used as a baseline to establish the amount of applied TP retained in the top 75 cm of soil since irrigation commenced. The mass balance indicates that 63 and 70% of wastewater-applied TP cannot be accounted for in the top 75 cm of soil in the field and forest, respectively. Because extensive records have been kept of effluent application rates and P concentrations, as well as harvested forage yields and nutrient contents, it is unlikely that imprecision in the values used in the mass balance can explain the magnitude of the unaccounted for effluent-applied P. Because the samples in the cropped field were taken at summit landscape positions, it is likely that surface runoff and subsurface lateral flow of effluent P, documented by other researchers, is partially responsible for the deficit of P in the 0-75 cm soil layer. Moreover, changes in the P-retention ability of the soil and the high hydraulic loading rate (irrigation plus natural precipitation of ~300 cm water per year) has probably caused leaching of P below the 75 cm depth. The capacity of the soil to assimilate and renovate effluent-applied P has been reduced as a result of irrigation with wastewater. M3Al and M3Fe results suggest podsolization may have occurred in both the field and the forest soil profiles, which could explain the lower P-retention properties of the surface soil layers. Since the soils are no longer acidic, the mobilization of Al and Fe is likely less significant than early in the system operation. Copper, which forms strong complexes with soil organic matter, also appears to have leached to a greater extent from the surfaces layers in the wastewater-irrigated forest. If leaching below 75 cm has occurred, the deep soils (> 200 cm) and distance (> 30 m below surface) to the permanent groundwater table suggest that the penetration of P into the subsoil of the forested area has had little impact thus far on groundwater water quality. Additional research is warranted to understand the long-term impacts of P application on the sustainability of land treatment as a viable means of recycling municipal wastewater effluent.