Gravel pit lakes form when gravel is excavated from below the water table of a phreatic or shallow confined aquifer. Typically many of these lakes are concentrated along naturally occurring sedimentary gravel deposits in areas where gravel is needed for construction. Most gravel pit lakes are relatively young features: most are less than 50 years old. The subject of this PhD thesis is to determine how gravel pit lakes change the hydrology and hydrochemistry of an aquifer, a watershed or a drainage basin. Hereto I studied gravel pit lakes in a fluvial freshwater setting of the Meuse Valley (the Netherlands) and gravel pit lakes excavated in ancient beach deposits, filled with brackish water along in the Adriatic coastal zone near Ravenna (Italy). One of the Dutch lakes is used for artificial recharge and drinking water production (DLV Lake) while some other gravel pit lakes are used for recreational purposes (swimming, sailing, scuba diving). The surface water of the lakes and other surface waters (wetlands, rivers) as well as groundwater up and downstream of the lakes was sampled and analyzed for major ion chemistry, trace elements and stable water isotopes. Chemical and water budgets were calculated. The excavation of many gravel pit lakes adds a large surface water area to a watershed. In the Dutch study site 71 lakes between the towns of Maastricht and Asselt add 20 km2 of surface water which is 0.26 % percent of the Dutch part of the Meuse watershed. In the Italian drainage basin thirteen lakes with a total surface of 684 hectares cover 6.6% % of the drainage basin. This increase causes a loss of freshwater since surface water evaporation rates in temperate and Mediterranean climates are usually higher than evapotranspiration rates of the pre-existing grassland and forest. The drainage pattern of a watershed changes in presence of gravel pit lakes causing fluctuations of the water table over a large area. In a low lying coastal zone, as the Italian study area, these fluctuations and the fact that the lakes form a constant head surface below sea level enhance salt water intrusion into the aquifer. Gravel pit lakes can be flow-through lakes where groundwater moves through the lake downstream towards a river or other draining feature (for instance a well field) or, alternatively, they may be in direct connection with a river. The gravel pit lakes that I studied in detail have in common that the water budget of the lakes is strongly determined by artificial drainage. In the Dutch DLV Lake, the artificial drainage is caused by pumping wells that extract water for drinking water production downstream of the lake. In the Italian case, the artificial drainage is induced by the land reclamation works that protect the low-lying land from flooding. Watersheds with multiple gravel pit lakes are more sensitive to changes in climate than watersheds without gravel pit lakes because surface water evaporation rates are more sensitive to changes in climate than evapotranspiration. Especially in groundwater fed gravel pit lakes, evaporated water is replaced by groundwater. Instead evapotranspiration of soil moisture in a watershed without gravel pit lakes, can increase only to certain extend as soil moisture is only fed by precipitation and not by groundwater flow. Water budget and conservative tracer modeling showed that because artificial drainage plays such a large role that changes in pumping rates needed to prevent flooding due to higher sea levels (The Italian study site) will affect evapo-concentration more than changes in surface water evaporation caused by climate change. Precipitation on the Italian gravel pit lakes is immediately mixed with brackish gravel pit lake water and can no longer recharge the fresh-brackish rainwater lenses in the upper part of the aquifer. Both the Dutch and the Italian gravel pit lake water has a high alkalinity, a high pH, and metal and trace element concentrations that differ from the groundwater in their respective watersheds. Differences do exist among the specific trace element concentrations, and their budgets in the lakes and the respective watersheds. This stems from the influence of sea water in the Italian case study and the specific soil chemistry of both settings. As and Ba, for example, show up in high concentrations in groundwater and gravel pit lake water in Italy but not in the Netherlands, where Ni, Zn and Al are more important. Differences in chemistry (Fe, SO4, HCO3, Ni etc. and pH) between gravel pit lake water and groundwater and variations along flow lines show that redox reactions in the soil near the gravel pit lakes occurred in both study sites. These reactions, enhanced by fluctuating water tables and/or denitrification of fertilized soils, have mobilized metals including Fe, Zn, and Ni and other elements such as Al and As. In part, these elements have been adsorbed again by the soil, as is the case for As in the Dutch site, in part they reach the gravel pit lakes where they precipitate on the lake bottom (for example, Fe, Zn, Ni, Al) and some elements remain (partly) in solution in the gravel pit lake water (e.g. As in the Italian lakes). The gravel pit lakes are strongly influenced by the land use and climate of their watershed. If circumstances change that would lead to less available oxygen either as DO or in NO3 or that would lead to a lower pH of the lake water, then the reactions that initially caused the deposition of the metals and trace elements on the lake bottom may be reversed. Metals and trace elements could go again into solution, possibly creating a toxic environment for plants, animals, and humans. These changes may be brought about by a change in land use, for example a reduction in the use of fertilizers, or a change in climate (less recharge of the aquifer), or slow leaching processes such as decalcification of the soil. On the other hand, an increasing eutrophication and primary production stimulated by high temperatures or less lake water circulation, would cause an increase in organic and fine grained material deposition to the lake bottom, which would help to fix the metals and trace elements in the lake bottom sediments. The rate of these processes may change over time since gravel pit lakes have formed only recently while land use and climate change play a role in their current and future evolution. The fixation of metals, C, nutrients and other elements in gravel pit lakes changes also the hydrochemistry of the estuary downstream of the lakes by preventing discharge of dissolved chemical elements into rivers and the sea. In order to assess and evaluate a watershed with gravel pit lakes for its safe use, it is necessary to monitor not only the lake water but also the groundwater, the water budget and the evolution of hydrochemical processes as climate and land use change.@en