| Ditches are ubiquitous features in altered landscapes. 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 great importance for controlling the timing and magnitude of terrestrially-derived contaminant exports to downstream water bodies.;My study examined the treatment function of ditches receiving domestic sewage effluents. The use of ditches for discharging wastewaters is common in regions where treatment systems are often non-existent. My study investigated the occurrence of in-stream contaminant retention in wastewater ditches, identified mechanisms responsible for contaminant retention, and implemented experimental modifications to the design of existing ditches to test whether treatment performance was improved.;The setting for the study was the town of Oxapampa, Peru where approximately two-thirds of wastewater generated is discharged to the Chorobamba River via earthen, vegetated ditches while the remainder is routed to the river via underground pipes. Dilution-corrected concentrations of E. coli, biochemical oxygen demand, total suspended solids and soluble reactive phosphorus 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.;Conservative solute tracer experiments revealed the important influence of ditch vegetation on transport characteristics such as residence time and transient storage, which have positive implications for improved contaminant retention and decreased export of sediment and E. coli. Deposition of silts and clays and organic matter accumulation were shown to be important drivers of improved phosphate retention by promoting sorption with benthic sediments.;Channel modifications to ditches were performed within a participatory framework that directly involved community members in planning, implementation and management. Two different modification approaches were tested: an open water flow design and an alternating subsurface flow/open water design. The latter design proved to be effective at sediment and E. coli removal and shows promise for treatment of water to irrigation standards. Reuse of treated ditch water should be promoted to prevent the continued eutrophication of the river. |
