Drinking water treatment residual use in urban soils: Balancing metal immobilization and phosphorus availability

- Concerns exist regarding health risks due to metal uptake by urban food crops.
- Soil amendment by drinking water treatment residuals (DWTRs) immobilizes metals.
- Alum-based DWTR amendment effectively reduced Cd and As uptake by B. pekinensis.
- Subsurface banded P fertilizer application enhanced plant uptake of fertilizer P.

Pollution in urban environments has raised concerns about the consumer safety of food produced by urban horticulture. Could a solid by-product of drinking water treatment provide the answer? Soil amendment with drinking water treatment residuals (DWTRs) has been shown to limit the uptake of metal contaminants, effectively immobilizing these elements within the soil; however, DWTRs possess a strong specific affinity for P and soil amendment with DWTRs can reduce P availability, thereby inducing plant P deficiencies and restricting growth. We conducted a glasshouse pot experiment to investigate the effect of alum-based DWTR amendment rate and P fertilizer placement method on Brassica pekinensis growth and As, Cd and P uptake under controlled conditions. Amendment of sandy loam soil A with 2-4 wt% DWTRs significantly (p < 0.001) reduced the Cd content of B. pekinensis tissues relative to untreated controls, with no significant difference in the P content of B. pekinensis tissues. Across all DWTR application rates examined (2-6 wt%), banded P fertilizer application resulted in 30-47% greater aboveground tissue P concentration compared to broadcast P fertilizer treatments (p < 0.05). Both the As and Cd content of B. pekinensis tissues were significantly (p < 0.001) reduced following 2-6 wt% amendment of sandy loam B with DWTRs. Results showed that a DWTR amendment rate of 2 wt% coupled with banded P fertilizer application resulted in the greatest B. pekinensis aboveground tissue biomass, along with a reduction in plant tissue Cd or As concentrations not significantly different from higher rates of DWTR amendment. Tissue P concentrations of B. pekinensis grown in soils amended with 2 wt% DWTRs were adequate for uninhibited plant growth. The use of DWTRs as an amendment to urban horticultural soils may provide a low-cost means of immobilizing trace metals without limiting plant uptake of applied fertilizer P. Additional study is required to quantify trace element release from DWTR-amended soils as a function of soil oxidation-reduction status.