Monitoring regional groundwater extraction: the problem

As hydraulic disturbances (signals) are propagated through a groundwater system two things happen: (1) the higher frequencies in the disturbance are filtered out by the physics of the system and (2) the disturbance takes time to propagate through the system. The filtering and time delays depend on the aquifer diffusivity. This means, for example, if one is observing a water table aquifer at some distance from where annual recharge is occurring, only the long-term average effect of the recharge will be transmitted to the observation point--the system filters out annual variations. These facts have profound impacts on what is feasible to monitor. For example, if one is concerned about the impact of pumping on a spring in a water table aquifer, where the pumping is more than 20 miles or so from the spring, there will be a long delay before the pumping impacts the spring and there will be an equally long delay before a long-term reduction in the pumping regime will restore the spring. The filtering by lower diffusivity groundwater systems makes it impossible to discriminate between the impacts of several major pumpers in the system and/or long-term climate changes.