Translocation and attenutation of wastewater phosphorus in streams

Abstract

Non-parametric statistical methods were used to identify the significance (at the 0.05 level) of differences between upstream (below each waste discharge) and downstream (at the end of each study reach) concentrations and loads of chloride and dissolved and available phosphorus in the water column. Regression analysis was used to define the equations of best fit for describing the variation of the study parameters under mean, high and low flow conditions along each stream reach. The slope of each regression line was tested for significance at the 0.10 level.

From this study, it was concluded that stream sediments in calcareous soil zones have a high and long term capacity for the removal of phosphorus in streams. Via this uptake an equilibrium was approached between the phosphorus in the sediments and in the water column which stabilized the dissolved phosphorus concentration in the stream.

Concentrations and distribution of the concentrations of sediment available phosphorus along each stream reach showed significant uptake of phosphorus as the impact of the waste discharge was ameliorated. Initial adsorption of phosphorus by the calcareous sediments was apparently followed by longterm mineralization and immobilization. High stream pH (<7.9) supported the probability of the reaction of phosphorus and calcite to form insoluble calcium phosphates. While the uptake mechanism appeared to be diffusion limited, the shortterm uptake capacity of the sediments exceeded the impressed load by a factor of sox to ten times. Continuous transport and renewal of the bed sediments enhanced the long-term removal-capabilities of the sediments.

Biologically available phosphorus followed patterns similar to those for dissolved phosphorus. Load attenuation showed significant exponential decay rates "ranging from -0.04 km⁻¹ to 0.36 km⁻¹. The lagoon effluent showed the highest mean rate (-0.19 km⁻¹) as its particulate fraction represented a source of dissolved phosphorus.

Attenuation of the concentration and load of dissolved phosphorus followed an exponential decay with stream reach. Significant reductions in phosphorus load downstream of each wastewater outfall were observed. The exponential decay rates ranged from -0.06 km⁻¹ to -0.39 km⁻¹. The rates of decay and significance of the regression relationships depended on the relative impact of the wastewater discharge and the magnitude of non-point source inputs. Observed rates increased with increasing influence from the waste discharge. Diffuse sources tended to mask the stream processes.

Bi-weekly field measurements made during the six month (May-october) study included stream discharge; dissolved and particulate phosphorus, chloride and calcium in the water column; and available phosphorus in the fixed bed sediments. The bed sediments were also analyzed for physical properties, mineralogy and chemistry.

This study addressed the trasnport and attenuation of wastewater phosphorus concentration and load in natural streams. Three stream systems in western New York were studied to assess their abilities to modify the applied phosphorus from wastewater discharges. The three small streams, lying in calcareous soil zones, were all developed in predominantly agricultural settings. Seven to ten sampling sites were established on study reaches ranging from 5 to 17.5 km below single waste discharge on each stream. The domestic waste discharges all received biological treatment; two included final polishing via sand filtration with the third adirect discharge from an algae pond.

Chloride was used as a baseline test for the behavior of a conservative material. Chloride was found to be conserved as the load accumulated linearly with reach. Rates of increase in load ranged from near zero to 58.4 kg Cl/km. Localized variations in the positive relationships were caused by diffuse sources and hydrologic and geomorphologic variations within the watersheds.