Exceedance of modern ‘background’ fine-grained sediment delivery to rivers due to current agricultural land use and uptake of water pollution mitigation options across England and Wales

Abstract

Excessive loss of fine-grained sediment to rivers is widely recognised as a global environmental problem. To address this issue, policy teams and catchment managers require an estimate of the ‘gap’ requiring remediation, as represented by the excess above ‘background’ losses. Accordingly, recent work has estimated the exceedance of modern ‘background’ sediment delivery to rivers at national scale across England and Wales due to (i) current agricultural land cover, cropping and stocking, and (ii) current land use corrected for the uptake of on-farm mitigation measures. This sectoral focus recognises that, nationally, agriculture has been identified as the principal source of fine sediment loss to the aquatic environment. Two estimates of modern ‘background’ sediment loss, based on paleolimnological evidence, were used in the analysis; the target modern ‘background’ (TMBSDR) and maximum modern ‘background’ (MMBSDR) sediment delivery to rivers. For individual (n=4485) non-coastal water bodies, the sediment ‘gap’ in excess of TMBSDR and MMBSDR, due to current land cover, cropping and stocking, was estimated to range up to 1368kgha−1yr−1 (median 61kgha−1yr−1) and 1321kgha−1yr−1 (median 19kgha−1yr−1), respectively. The respective ranges in conjunction with current land cover, cropping and stocking but corrected for the potential impact of on-farm sediment mitigation measures were up to 1315kgha−1yr−1 (median 50kgha−1yr−1) and 1269kgha−1yr−1 (median 8kgha−1yr−1). Multiplication of the estimates of excess sediment loss corrected for current measure uptake, above TMBSDR and MMBSDR, with estimated maximum unit damage costs for the detrimental impacts of sediment pollution on ecosystem goods and services, suggested respective water body ranges up to 495 £ha−1yr−1 and 478 £ha−1yr−1. Nationally, the total loss of sediment in excess of TMBSDR was estimated at 1,389,818tyr−1 equating to maximum environmental damage costs of £523Myr−1, due to current structural land use, compared to 1,225,440t yr−1 equating to maximum damage costs of £462M yr−1 due the uptake of on-farm sediment control measures. The corresponding total loss of sediment in excess of MMBSDR was estimated at 1,038,764tyr−1 equating to maximum damage costs of £462Myr−1, compared with 890,146tyr−1 and £335Myr−1 correcting excess agricultural sediment loss for current implementation of abatement measures supported by policy instruments. This work suggests that the current uptake of sediment control measures on farms across England and Wales is delivering limited benefits in terms of reducing loadings to rivers and associated environmental damage costs.