ASSESSING THE ROLE OF A LIMESTONE QUARRY AS SEDIMENT SOURCE IN A DEVELOPING TROPICAL CATCHMENT

Impact assessments on river systems of the combined effect of bed and suspended sediment loads from quarries are difficult to find. In this study, bed and suspended loads were measured to determine the impact of a 20-ha limestone quarry on the river system of its 5,000-ha steep, diverse land use/land cover but mostly forested catchment. A network of hydrologic and sediment monitoring instruments was deployed over the catchment during two separate study periods when sediment loadings were measured from captured storms. Results showed that the quarry stood to make a disproportionately large contribution to the catchment's estimated 2.1 Mg ha(-1) yr(-1) suspended sediment load. Large storm events contributed most of the loadings with five events supplying 92% of total loadings at the outlet. A paired method approach to compare suspended sediment loads between two subcatchments showed that during eight storm events, the quarry yielded between 2 and 49.2 Mg ha(-1) per event, whereas the forest never yielded more than 0.1 Mg ha(-1). Furthermore, the contribution of sediments from the quarry to bed load was more than 75% at a section located 1.2 km downstream. Future management activities to reduce sediment and bed loads, not only from this catchment but also from all others with similar land use/land covers, should focus on improving quarry operations. Copyright (C) 2014 John Wiley & Sons, Ltd.