Runoff water quality and vegetative establishment for groundcovers on steep slopes

Abstract

Construction sites can be a major source of sediment, a common pollutant in surface waters, because steep slopes are often bare during active construction. The use of mulch and polymers has been found to reduce erosion rates and improve runoff water quality on construction sites. Different erosion control methods on steep slopes (2:1) consisting of straw, straw plus 37 kg ha⁻¹ (33 lb ac⁻¹) linear anionic polyacrylamide (straw + PAM), and excelsior blankets were evaluated based on runoff water quality, vegetative establishment, or both at six construction sites. Plots (3 to 9 m [10 to 30 ft] wide by 9 m [29.5 ft] long) were established on cut slopes. Runoff was collected at four of six sites by installing plastic barriers to direct a portion of runoff into containers, later analyzed for turbidity and total suspended solids (TSS) in the laboratory. Large differences in water quality were often not statistically significant for individual storms due to high variability. However, when data were averaged across all storm events, the straw + PAM treatment reduced mean runoff turbidity at three of four sites and mean TSS at two of the four sites compared to straw alone. Maximum reductions in turbidity and TSS were 81% and 56%, respectively. Runoff from the excelsior plots had significantly lower turbidity and TSS than straw plots in one of three sites. Compared to straw + PAM, excelsior had significantly higher turbidity at two of three sites and higher TSS at one of three sites. All differences among treatments for individual storm events occurred within the first three events. Vegetative cover was determined in the field, and aboveground vegetation was harvested to determine biomass production. Neither vegetative cover nor biomass were affected by treatment, and average cover was 60% or less for five of the six sites. Rainfall patterns were largely responsible for vegetative growth, with heavier rainfall soon after seeding tending to reduce cover.