Evaluation of Biosolids for Ecological Restoration of Degraded Soil: A Field Study.

Healthy soils of high quality are the base of ecosystem support.  Soil quality is essential for proper functioning of terrestrial and aquatic ecosystems.  Several research and demonstration studies have shown that biosolids are an effective soil amendment to restore degraded land and sequester contaminants in soil thereby reducing/eliminating their risk to human and ecological health.  An important task of restoration of degraded urban land is accurate evaluation of improved soil quality and its effect on human health and ecosystem function before and after restoration. Evaluation of restored soils should be based on soil quality indices including biological availability/exposure of contaminants of concern to ecological receptors.   This study evaluated the use of biosolids for ecological restoration.  Field scale runoff plots of 3 x 9 m, with 4 replications of 5 treatments were installed in the Calumet region of Chicago, Illinois, USA.  Treatments were 2 different rates of biosolids, vegetative compost, a custom mix of biosolids, iron oxide  (Fe_ox ) rich water treatment residual (WTR)and biochar, and top soil as a control.  The purpose of the WTR and biochar was to reduce runoff P and microconstituent availability.  Rainfall runoff from experimental plots was collected for 2 years. A mixture of native forbs and grasses were planted on the plots. Vegetation was monitored, collected and analyzed.  Earthworm bioassays were conducted in a controlled laboratory experiment from plot soils.  (1.) Runoff analysis showed that all biosolids treated plots had higher levels of P concentrations (1.3 – 5.5 mg/L) compared to top soil and compost treated plots (0.1 – 1.6 mg/L) over the 2 years, that total P from sediment loss was highest in the first year, with biosolids treated plots having higher values (60 – 100 kg /ha) than compost or top soil.  This dramatically decreased as soon as the vegetative cover was established.  Flow weighted soluble P increased with precipitation events significantly in the second year, with all biosolids treated plots contributing significantly more than top soil or compost.  Soil analysis showed that biosolids treated plots increased total organic carbon levels to 1.4 -1.5 greater and total N 1.9 to 2.5 greater that the control or compost treatment.   Soil levels of plant nutrients were increased significantly for Cu, Fe, K, P and Zn.  Select soil trace element metal(loids) did not exceed USEPA subpart 503c regulatory limit. Thus, biosolids imparted benefits to soil that are conveyed by establishing plant cover while posing little risk to ecological receptors.  Earthworm bioassay showed no adverse impact of the biosolids treated plots on mortality or reproduction. Plant biomass increased in biosolids plots, and total number and diversity of species were improved. Pharmaceutical and personal care products measured in the runoff were very low (ng L^-1) and were detected in all plots. The benefits of the biosolids application outweighed potential risks, indicating biosolids is desirable for ecological restoration of degraded soil.