Recognizing ditches as globally important interface ecosystems: An evaluation of their potential for mitigating anthropogenic impacts to receiving water bodies

Ditches are ubiquitous features in altered landscapes. Their presence grows in proportion to the expansion of anthropogenic land use and as a consequence, ditches have become an inevitable component of ecosystems around the world. In developed nations, their primary purpose is to route water for irrigation or to improve drainage from agricultural fields and roadways. Drainage ditches are equally pervasive in many parts of the developing world; however, in the absence of wastewater treatment facilities, drainage ditches often serve as the primary transport conduits for untreated human sewage. Although ditches are designed to be efficient at moving water across the landscape, there is growing evidence that ditches provide services beyond basic water transport. The position of ditches in the landscape lends them unique importance as interface ecosystems, connecting terrestrial and aquatic environments and controlling the timing and magnitude of terrestrially-derived contaminant exports to downstream water bodies. This study addresses the question of whether ditches demonstrate the potential to mitigate contaminant loads to receiving bodies. A case-study evaluating contaminant retention in wastewater drainage ditches in Peru is presented. Dilution-corrected concentrations of biochemical oxygen demand, phosphate and E. coli bacteria exported from ditches were found to be significantly lower than the same effluents discharged via underground pipes, indicating that transport in ditches is not conservative. The findings from this study are coupled with those from other ditch systems to develop a general understanding of where the greatest opportunities exist for capitalizing on drainage ditches for improved water quality, as well as to identify major pitfalls or limitations. This overview is a call for a re-evaluation of how we perceive ditches in the landscape with a focus on how ditch ecosystems might be designed and managed to enhance their water treatment function.