GROUNDWATER RESIDENCE TIMES AND HYDRAULIC RESPONSES: IMPLICATIONS FOR SUSTAINABLE DEVELOPMENT OF REGIONAL GROUNDWATER FLOW SYSTEMS

The long timescales of groundwater associated with most regional flow systems have created a challenge in understanding how these systems can be sustainably developed. Residence times of these waters have been used to assess whether groundwater withdrawals are sustainable. However, the link between residence times and the hydraulic responses of these systems, which controls the water levels and the amount of water in storage, is exceptionally challenging to quantify and with much at stake; assuming groundwater storage volumes are related to climate conditions at periods equal to mean aquifer residence times could result in labeling much of the Earth’s groundwater as nonrenewable. Here, we examine the relationship between residence times and hydraulic response times for 31 regional aquifers. Residence times for these aquifers varied from 0.7 a to 10 Ma while aquifer response times varied from 1.6 years to 6.9 Ma. These results show that groundwater system re-equilibration following changes in the hydraulic boundary conditions cannot easily be predicted from the residence times. A significant correlation between residence time and hydraulic response time exists but explains less the 57% of the variance across the 31 aquifer systems. Other factors including (paleo)climate conditions, aquifer system flow length and hydraulic conductivity influence both residence time and hydraulic response time, possibly helping to explain the existence of a statistical relationship between residence and retention times. Modern hydraulic heads should be >95% re-equilibrated from a change in hydraulic conditions occurring at the mean residence time in the majority of aquifers systems. Groundwater storage in excess of that expected with modern boundary conditions will occur in many aquifers due to periods of higher recharge at times less than the residence time of the aquifer. In other cases, aquifers containing Pleistocene water have storage volumes that reflect modern boundary conditions. Retention of fossil groundwater can occur without persistence of excess groundwater storage associated with past climates.