Water and (bio)chemical cycling in gravel pit lakes: A review and outlook

The world produces 1.7 × 108 metric tons of gravel and sand per year (USGS, 2015) creating many gravel pit lakes that change the morphology and drainage pattern of catchments. Gravel pit lakes abruptly intersect the geologic layering creating an environment where surface and groundwater will interact and where elaborate food webs can develop. Here we preview previous work on gravel pit lakes and compiled a comprehensive hydrochemical database to compare the chemistry of gravel pit lake water with other types of surface and groundwater. Water budget calculations confirm that gravel pit lakes cause freshwater loss in temperate and Mediterranean climates where surface water evaporation is larger than the actual evapotranspiration of vegetated land that was replaced by the gravel pit lakes. Groundwater fed gravel pit lakes where evaporated water is replaced by groundwater are especially sensitive to climate change.The gravel pit lakes included in this review have a relatively low acidity and high alkalinity most likely caused by weathering and leaching of carbonates in the catchment. The inflow of groundwater is a key process in gravel pit lakes with important consequences. The creation or presence of the gravel pit lakes may induce fluctuation of the up-stream water table which enhances groundwater flow and redox reactions in the soil. Groundwater rich in dissolved elements typically meets more alkaline water in gravel pit lakes enhancing the precipitation of metal oxides, calcite and other composite minerals including phosphorus (P), calcium (Ca) and carbon (C). Gravel pit lakes provide many different ecological habitats increasing the biodiversity in typically an agricultural or urban setting. Plant and animal species observed in gravel pit lakes consists of phytoplankton, zooplankton, micro plankton, macrophytes, fish and birds similar to natural lakes but the fact that gravel pit lakes may be only groundwater fed, or instead in open contact with rivers causes large variations between the ecosystem of different lakes. Plants and animal species take part in the chemical cycling of gravel pit lakes by, among others, uptake of atmospheric carbon dioxide (CO2) and nitrogen (N2), of dissolved compounds including bicarbonate (HCO3), iron (Fe) and manganese (Mn); of elements including phosphate (P) and Fe from lake sediments, and carbon mineralization and burial. Gravel pit lakes may contribute to denitrification of groundwater as N is consumed by plankton, but they may also enhance the mobilization of soil-bound compounds like potentially toxic (trace) metals released from aquifer sediments. The creation of gravel pit lakes provides more available sites for carbon burial but once deposited on the lake bottom, metals and other elements may be released again due to redox cycling, influenced by climatic or land use change. Gravel pit lakes are water bodies of recent formation and so far only a few different settings have been studied in detail compared to other types of natural- and man-made lakes. From this review it is evident that gravel pit lakes are hydrochemically most similar to so called 'marl lakes' or 'nutrient rich' lakes. Key areas for further research include the study of gravel pit lakes in other settings to better separate the similarities and differences between natural and gravel pit lakes. Also the feedback mechanisms between change in land use and climate, ground- and lake water chemistry ecological functioning and use of the gravel pit lakes need to be addressed.