Effect of vegetated filter strips on transport and deposition rates of Escherichia coli in overland flow in the eastern escarpments of the Mau Forest, Njoro River Watershed, Kenya

Abstract

The fate and transport of Escherichia coli (E. coli) in lotic waters through vegetated filter strips (VFSs) was evaluated in a field model pasture, utilizing VFSMOD Windows along with direct pathogen testing. This study assessed effects of VFS on transport and deposition rates of E. coli in lotic overland flow waters. The VFS measured 44 m long by 40 m wide, covering an area of 1584 m² and land slope of 15 %. Cowpat was applied onto the model pasture and washed by overland flow into the VFS. The 4-methylumbelliferyl β-D-glucuronide substrate confirmed the identity of E. coli prior to cowpat application and after isolating them from soil using centrifugation and membrane filtration techniques. Napier grass root system recorded the highest recovery rates of E. coli at 99.9 % along the length of VFS III. This efficiency reduced significantly (p < 0.05; df = 29) to 95 % in Kikuyu grass and 75 % in Couch grass-Buffer grass. The data demonstrated similarity in transport of manure-borne E. coli and organic carbon (OC) through all the simulated VFS. These results indicated that OC could be used as a true natural tracer of manure-borne E. coli, a pollution indicator organism of lentic and lotic surface waters provided the OC release kinetics from cowpat were similar to that of E. coli kinetics. Thus, efficient filtering to reduce E. coli concentrations and load in overland flows requires managing combined grass species, agro-pastoral systems models and dispersed or preferential flows to enhance surface water quality standards.

Keywords: Escherichia coli; Fate and transport; Lotic surface water; Mass balance; Micro-biome; Overland flow rate; VFS.

Fig. 1

Study sites at Tatton Agriculture Park, in the eastern escarpment of the Mau Forest, Njoro River Watershed, Kenya (courtesy of Zack Ogari, KMFRI, Kenya)

Fig. 2

a Kikuyu grass field plot, b Napier grass field plot

Fig. 3

Site layout design contour grids, longitudes and latitudes coordinates and sampling sites for vegetated filter strip experimental field plots in a randomized complete block design; block A (C1-30: VFS I, N10K20: VFS II, K10N20: VFS III); block B (C2-30: VFS I, K10N20: VFS III, N10K20: VFS II); and block C (C3-30: VFS I, N10K20: VFS II, K10N20: VFS III). Key to Fig. 2 Kikuyu grass (K), Napier grass (N), subscript numbers in meters (10, 20) and control (mixed grass C1–C3)

Fig. 4

The observed and predicted mean $(\text{SEM}\pm \sigma \overline{x})$ release rates of nutrients and E. coli along the length of VF

Fig. 5

Predicted and observed E. coli removal effectiveness and efficiency by vegetated filter strips averaged across three replications, August 2013 to December 2014

Fig. 6

Relationship between settling velocity of E. coli and cumulative density function through Napier grass, Kikuyu grass and Couch grass–Buffer grass along the length of VFS I, II and III, respectively, averaged across three replications, August 2013 to December 2014

Fig. 7

Hyetograph and hydrograph relationships and sediment delivery ratio during rainfall events through VFS averaged across three replications, August 2013 to December 2014

Fig. 8

The mean overland flow rates (Q) through the Couch grass–Buffer grass along the length of VFS I, Kikuyu grass along the length of VFS II and Napier grass along the length of VFS III at 95 % confidence level averaged across three replications, August 2013 to December 2014

Fig. 9

Relationship between normalized E. coli concentration and total organic carbon (OC) concentrations in overland flow through Napier grass, Kikuyu grass and Couch grass–Buffer grass; note E. coli—concentration of E. coli in overland flow; E. coli₀—the initial concentration of E. coli in cowpat; also shown is the linear regression line

Fig. 10

Temporal distribution of runoff intensity (m³s⁻¹) in relation to total coliform, fecal coliform and E. coli concentrations in Couch grass–Buffer grass (VFS I), Kikuyu grass (VFS II) and Napier grass (VFS III) in dry (January–March 2014) season, wet (April–August 2014) season and short rainy seasons (September–March 2013) and (September–December 2014) seasons averaged across three replications, August 2013 to December 2014