Composition and design of vegetative filter strips instrumental in improving water quality by mass reduction of suspended sediment, nutrients and Escherichia coli in overland flows in eastern escarpment of Mau Forest, Njoro River Watershed, Kenya

Abstract

This study assessed the effect of vegetative filter strip (VFS) in removal of suspended sediment (SS), total nitrogen, total phosphorus and Escherichia coli (E. coli) in overland flow to improve receiving water quality standards. Four and half kilograms of cowpat manure was applied to the model pasture 14 m beyond the edge of vegetated filter strip (VFS) comprising 10-m Napier grass draining into 20-m Kikuyu grass (VFS II), 10-m Kikuyu grass draining into 20-m Napier grass (VFS III) and native grass mixture of Couch–Buffel (VFS I-control). Overland flow water samples were collected from the sites at positions 0, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25 and 30 m along the length of VFSs. E. coli removal by Napier grass VFS was on the order of log unit, which provided an important level of protection and reduced surface-flow concentrations of E. coli to below the 200 (CFU 100 mL−1) recommended water quality standards, but not for nutrients and SS. The Napier grass showed highest efficiency (99.6 %), thus outperforming both Kikuyu grass (85.8 %) and Couch–Buffel grasses VFS (67.9 ± 4.2 %) in removing E. coli from overland flow. The low-level efficiency of native Couch–Buffel grasses in reducing E. coli in overland flow was because of preferential flow. Composition and design of VFS was instrumental and could be applied with a high potential of contracting the uncertainty in improving water quality standards through mass reduction of SS, nutrients and E. coli load in watersheds.

Prairie strips reduce fecal indicator bacteria concentrations in simulated runoff

“…Designing the width of prairie strips requires an estimate of the variation of concentration with distance. Contaminant removal in VFS and urban stormwater treatment systems is frequently estimated using first‐order decay kinetics (Elliott & Trowsdale, 2007; Lerch et al, 2017; Olilo, Muia, et al, 2016; Olilo, Onyando, Moturi, Muia, Ombui, et al, 2016; Olilo, Onyando, Moturi, Muia, Roegner, et al, 2016; Wong et al, 2006) due to its apparent behavior to be weighted to the influent end of the treatment. Inspection of concentration data from Coyne et al (1998), for example, shows that surficial concentration reductions through VFS closely follow a first‐order decay model, written as:where C 0 is the concentration of a contaminant initially entering the prairie strip or VFS, x is the distance from the entrance to the downstream sampling location, C is the concentration at location x , and a is a lumped removal parameter.…”