Phosphorus Reduction In Runoff Using a Steel Slag Trench Filter System.

Excessive concentrations of phosphorus (P) can cause deterioration of surface waters by eutrophication. Previous research has demonstrated that P sorptive industrial byproducts loaded into structures placed into hydrologically active drainage have the ability to sequester dissolved P from targeted areas. The objective of this study was to evaluate the performance of a steel slag trench filter system developed for P reduction in runoff under field conditions. The experimental design for the study was a 2 x 2 factorial with factors of filter material (6 mm steel slag or 13 mm washed river gravel) and triple superphosphate fertilizer (P applied or no P applied) replicated within irrigation zones covered with ‘Astro’ bermudagrass [Cynodon dactylon L. (Pers.)] mowed at 38 mm. Each irrigation block consisted of four plots that were 6.1m wide with a uniform 5% slope that measured 24.4 m long. Four plastic containers (volume = 178 L each) were placed in a 5.2 m × 1.2 m trench dug in the middle of each plot perpendicular to the slope to accommodate filter materials. Runoff caused by either natural rainfall or irrigation was collected for comparison of P concentrations before and after filtration. Flow rates were determined using ultrasonic depth detection devices as runoff flowed through calibrated Parshall flumes. From March through September 2012 a total of 14 runoff events were studied. Runoff filtered by steel slag contained from 17% to 43% less dissolved P than runoff filtered through an inert gravel control. The P concentrations in runoff from the fertilized treatment were always greater than the unfertilized treatment.  The difference between P concentrations from the fertilized and unfertilized treatments declined with each runoff event from 9.48 mg L^-1 in the initial event after fertilization to 0.20 mg L^-1, 120 d and nine runoff events later.