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.

Keywords: E. coli; Mass reduction; Nutrients; Overland flow; Suspended sediment; Vegetated filter strips.

Fig. 1

Sampling site at Tatton Agriculture Park, eastern escarpment of the Mau Forest, Njoro River Watershed, Kenya (Source: CNES/Astrium and Google Inc, 2016)

Fig. 2

Layout design of VFS in field plots II, III and I at TAP, eastern escarpment of Mau Forest, Njoro River Watershed. Block III: VFS I (Couch–Buffel grasses = control); VFS II (N = Napier grass, K = Kikuyu grass); VFS III (K, N); Block II: VFS I (Control); VFS III (K, N); VFS II (N, K); and Block I: VFS I (Control); VFS II (N, K); VFS III (K, N); CT = collecting troughs

Fig. 3

Observations and predictions of SS, TN, TP and E. coli mass transport in overland flows at VFS length exit of Couch–Buffel, Napier and Kikuyu grasses from August 2013 to December 2014

Fig. 4

Effect of VFS length on mass transport of SS (mg ha⁻¹) in overland flow in Njoro River Watershed from August 2013 to December 2014

Fig. 5

Temporal variation of VFS model parameters in overland flows in Njoro River Watershed from August 2013 to December 2014. Note Pearson product moment correlation coefficients (r) and root mean square error (RMSE) for cumulative overland flow in VFS; r, RMSE and the Nash–Sutcliffe are given for E. coli concentrations in the overland flow from August 2013 to December 2014

Fig. 6

Effect of VFS lengths on mass transport of TN (mg ha⁻¹) in overland flow in Njoro River Watershed from August 2013 to December 2014

Fig. 7

Effect of VFS lengths on mass transport of TP (mg ha⁻¹) in overland flow in Njoro River Watershed from August 2013 to December 2014

Fig. 8

Effect of VFS lengths on mass transport of E. coli (CFU 100 mL⁻¹ ha⁻¹) in overland flow in Njoro River Watershed from August 2013 to December 2014

Fig. 9

Effect of VFS relative density on mass reduction efficiency (%) of E. coli by different grasses in Njoro River Watershed from August 2013 to December 2014

Fig. 10

Mean percentage root zone retention of E. coli by VFS from August 2013 to December 2014