Conjunctive Use and River Enhancement

Pumping groundwater into rivers to augment streamflow is a form of conjunctive use. However, rather than minimizing adverse effects of water use, it directly enhances streamflows.

Conjunctive use : Combined use of surface and ground water systems to optimize resource use and minimize adverse effects of using a single source

Groundwater Storage

The concept of withdrawing groundwater storage to improve water resources management is not new. Withdrawal during dry periods, balanced by replenishment during wet periods, is, in many situations, a long-term and sustainable approach to water resources management (Alley and Leake, 2004). Careful manipulation of groundwater storage can produce streamflow benefits with little or no environmental impact, and, in some instances, can provide additional water supply. This approach is most applicable in a maritime or mountainous climate where there is significant seasonal run-off and associated groundwater recharge. It can also offer significant advantages over constructed surface storage impoundments.

Source of Water to Wells

When a well is initially pumped, water is withdrawn from the pore spaces in the aquifer. The storage coefficient of an aquifer is a measure of the volume of water within the pore spaces of the formation. As pumping continues over time, the influence of the well extends outward from the well to hydraulic boundaries of the aquifer system, eventually establishing an equilibrium within the recharge and discharge continuum of the aquifer system as a whole. Accordingly, a long-term continuous groundwater withdrawal generally causes a permanent change to the recharge-discharge equilibrium of an aquifer.
Streams and rivers are commonly important hydraulic boundaries to an aquifer, and there are many examples where continuous groundwater pumping has affected streamflows. In some cases pumping induces river water into the aquifer. In other cases, pumping reduces the volume of groundwater discharge that would otherwise flow into a river as baseflow.

Time-Varying Response to Pumping

During the first 100 days of pumping, more than 50% of the water withdrawn from the well was withdrawn from aquifer storage, with the remaining 50% reflected as a reduction to stream baseflow. After pumping stopped, both aquifer storage and residual effects to stream baseflow recovered to a within 10% of a pre-pumping condition in about 200 days. This plot illustrates that pumping does have a relatively immediate effect on streamflows, but that the magnitude of the effect varies over time. The figure also illustrates that seasonally pumping groundwater and discharging it directly to a stream can increase streamflows, even if an aquifer is in hydraulic continuity with the stream.

Seasonal or non-continuous pumping returned directly to a stream can provide a net increase in surface flows over current conditions. During the wet season, pumping of the aquifer does not occur, and the aquifer is allowed to recharge naturally, recovering storage capacity to sustain another cycle of augmentation. The recovery period is critical to minimizing long-term effects to the recharge-discharge equilibrium from pumping. Variability in the magnitude and rate of recharge to the aquifer during the wet season should be investigated, and could be a controlling factor for dry season pumping.

Artificial recharge of the aquifer during the wet season could potentially be used to enhance the wet season recharge cycle, depending on specific circumstances.

The frequency of cycling depends, in most cases, on the recharge capacity of the aquifer system and frequency of low streamflow levels. Some aquifer systems may be able to sustain significant level of augmentation year after year, while others may require longer periods to recover. Similarly, augmentation of streamflows may not be necessary every year, or may be more intensive on some years compared to others. In this regard, direct groundwater augmentation provides a level of flexibility and control that may not be possible with a more conventional surface storage approach.