b'WASH SYSTEMSClear water pond: water from settling ponds is stored here forWashing in order to remove fines (silt- and clay-sized particles) re-use in the wash plant off coarse aggregateIn order to maintain a satisfactory water level in the clear pond,in the spent wash plant settles out in the ponds, resulting in essen-make-upwatermayberequiredduringperiodsoflowrainfalltially sediment-free water that is returned to the clear pond. and high evaporation. Make-up water can be introduced into the loop through onsite pumping wells and/or surface water sources.On sites where the ponds are not lined, the settled fines them-Insomeinstances,thefreshwaterpondmaybehydraulicallyselves may act to seal up (line) the base of the pond. As the ponds connectedtoashallowgroundwateraquifer.Apermittotakecapacities are diminished, however, they can be dredged out and the water (PTTW) is normally required for the make-up water source.excavated material can be used for rehabilitation purposes at the site. For below water pits and quarries, a PTTW for the purpose of dewatering may allow operations to utilize water from the sumpOn occasion, suspended sediment may enter the clear pond, for for the purposes of aggregate washing. example, during high intensity storm events. The occasional presence of a relatively low sediment load in the return water to the clear pond As the wash water moves through the settling ponds, much ofis not a major concern since a) there is no off-site discharge, b) the the suspended solids settle out relatively quickly. The settlementsediment will further settle out in the clear pond, and c) the sediment rate is actually dependent on many factors, including velocity andis a natural component of the bedrock/sand and gravel subsurface.particle shape. For this reason, settling ponds come in a variety of configurations and sizes, from sinuous to square or rectangular,Additionally, the deposition of wash water and fines in the ponds and are designed to suit site conditions. The suspended sedimentdoes not affect the chemical or physical quality of the subsurface soils or groundwater since the aggregate materials and water typi-cally originate from the site.It is important to note that as wash plants become more techno-logically advanced (e.g., hydrocyclones, clarifiers, flocculent systems, etc.), the industry is better utilizing the resource and reducing water adapting to better suit the industry and the environment.loss.As a result, the design of closed loop wash plants is always Craig Leger is an environmental consultant with WSP Canada.MARKEPLACETECTECGPSAERIALClean water dischargeWINTER 2020AVENUES 31'