Understanding of Aquifer Systematics Can Aid Water Planning and Policy
W. Peter Balleau, CPG-2716 Abstract
Technical methods can promote safe planning and policy for aquifer development projects. These meth- ods have long been applied to calculate the area of surface water capture supporting pumping from new wellfields. The foreseeable capture of responsive surface water is the final source that offsets the stress of well development and leads to sustainable pumping. Such classical methods address the risks of wells going dry or of beneficial water being left unused. Simplified alternative methods involving footprints or indexes of recharge to pumping are unable to reveal the possibly undesirable outcomes.
Keywords: Aquifer Sustainability, Capture, Water Policy, Area of Influence Introduction
Water-planning agencies often request hydrogeologists to determine sustainable aquifer conditions. However, our an- swers vary. There seems to be no handy rule-of-thumb for assessing aquifer sustainability. Simplifications such as to “pump the recharge” fall short, and overall effective aquifer use and forecasting remains elusive. But effective policy on aquifer use and impacts is increasingly sought, and stands as an issue of critical importance, given that groundwater is the material extracted in highest volumes from the Earth’s crust and is used as the primary source of freshwater by over two billion people (Zektser and Everett, 2004).
Despite the challenges, the basic systematics of the re- sponse to a new stress in aquifers are well-established. Groundwater use is eventually balanced by capture of adja- cent surface water sources that offset the pumping amount, while pumping rates are limited by practical pumping water levels (Theis 1940; Lohman 1979; Konikow and Leake 2014). This commentary focuses on the critical role of the wellfield’s area of influence. The area necessarily expands until it incor- porates adjacent surface water sources that offset the stresses imposed by the users of a given wellfield.
The core items of concern to water policy and planning 8 TPG •
Oct.Nov.Dec 2021
commonly include: a) the practical amount of wellfield withdrawal for use, b) the wellfield area of influence, c) the change in aquifer water levels, and d) the surface water impacts.
Aquifer dynamics require a physical response to the stress caused by pumping. An expanding drawdown cone releases water from storage (aquifer depletion) while the cone gener- ally expands radially from the well toward any hydraulically- connected boundary features. Those boundaries are where the groundwater and surface water intersect. The altered flux at the boundary features is called capture or surface water depletion. The majority of well abstractions are expected to be sourced in the long term by captured surface water (Koni- kow and Leake 2014). Declining water levels do not necessar- ily imply “overdraft.” Some decline is a necessary part of the transition toward a rebalanced system.
A possible constraint on drawdown is the practical pump- ing water level (PWL). That limitation can arise from econom- ics, well construction, or the shallow floor of the aquifer. Well- field yields decline if a PWL constraint is reached. Declining yields also do not necessarily imply “overdraft.” Sometimes, a new steady state is pending at lower pumping rates.
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