Metal accumulation in an artificially recharged gravel pit lake used for drinking water supply

• Flow-through gravel pit lakes are safe to use for artificial recharge and water production.
• Acidifying redox reactions upstream from the gravel pit lake mobilize metals.
• Gravel pit lakes are a sink for metals.
• Nutrient, metal and other chemical budgets in aquifer, lake water and sediment are complexly related.
• Artificial recharge in gravel pit lakes for water production has certain advantages over other methods.

Gravel pit lakes offer a variety of uses after excavation has ceased. One of those uses is the storage, infiltration, aquifer passage, and production of drinking water. We have investigated such a gravel pit lake in The Netherlands that is a state of the art drinking water production facility. The gravel pit lake is a flow-through lake, fed naturally by groundwater (~ 17%) and rainwater (~ 6%), and artificially with Meuse River water (~ 77%). The average concentrations of Cd, Cr, Cu, Ni, Pb and Zn in the lake's bottom sediments have increased over a 10 year period. Acidifying redox reactions caused by lowering of the water table and farmland fertilization upstream from the lake explain the mobilization of metals in the soil and subsequent transport with groundwater towards the lake. Dissolved metals (Al, Cd, Cr, Cu, Fe Mn, Ni and Zn) and PO43 − flow with the groundwater towards the lake, where they interact with oxygen-rich and alkaline water to (co)precipitate as Fe, Mn and Al oxides. The chemistry of the gravel pit lake water is determined by a complex interplay between the input of NO3− and PO43 −, soil composition up stream, biochemical processes in the lake that supply organic material, mixing processes (artificial and natural) that determine the amount of stratification and oxygen, and redox cycling processes in the bottom sediments that influence the concentration of metals and P in the lake water. Gravel pit lakes are young compared to natural lakes and the long term influence of these lakes is still largely unknown. This study confirms that they can form a sink for metals and thus influence the metal budget of a watershed. Compared to other artificial recharge methods, infiltration in a gravel pit lake along a river has certain advantages in that the lake already exists, the transport distance for the water to be infiltrated is short, the pretreatment of the infiltrated water is simple, and the long residence time of the water in the lake and the lake bank filtration attenuates quality variations of the infiltrated river water. For drinking water production or other usage of gravel pit lakes, an integrated monitoring and management of land use, ground- and surface water as well as lake bottom sediments are needed.

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