Using Groundwater Responses to Infer Recharge - Part 5

By D Armstrong and K Narayan

Analytical methods of assessing the response of groundwater levels to a range of factors, including elastic (barometric and tidal) influences in confined aquifers and recharge to unconfined aquifers due to infiltration of rain and other surface water, are presented. Responses in a confined aquifer to distant recharge events and the associated time lag is discussed. Also covered are responses to changes in storage volume resulting from direct recharge at the outcrop of an unconfined aquifer system both seasonally and on a single recharge event basis. Worked examples and case histories are used to illustrate methods of estimating the amount of recharge at different sites within a catchment. The application of vertical cross-sectional flow nets to the estimation of recharge is presented in the context of recharge/discharge profiles.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Jan 1, 1998
• ISBN: 9780643106062

Book preview

1 INTRODUCTION

Recharge to a groundwater system represents the addition of water to the body of water already stored in the ground. The volume of water stored is increased, therefore it is reasonable to expect a rise in water level in the area receiving recharge.

The rise in water level should be proportional to the ‘depth’ of water recharged with the constant of proportionality equal to the specific yield (Sy) of the material containing the water table.

Δh = R/Sy         (1)

where Δh = rise in water level (m)

R = recharge as a depth of water (m³ m−2)

Sy = specific yield of water-table aquifer.

This change in water level will only occur when the recharge water reaches the water table, and may take only a few minutes or hours for a shallow groundwater system in which the water table is within a few metres of the surface and the unsaturated zone is relatively permeable. Where the water table is overlain by relatively impermeable materials, such as clays, or is very deep, it may take months or even years for the recharge water to reach the aquifer and cause the water level to rise. An outstanding example of the latter case is described by Allison et al., (1990) for the Western Murray Basin where it is estimated that it would take 75 years for an increase in recharge from almost 0 mm yr−1 to 20 mm yr−1, caused by clearing of native vegetation, to be observed at a water table at a depth of 25 m.

The interpretation of water level rises must be tempered by the knowledge that recharge is only one of many possible causes and, without a detailed long-term record of water levels at a site and information about nearby water-related activities it may not be possible to separate recharge events from other causes such as recovery from pumping in a nearby well, or rise in storage level in an adjacent reservoir.

Groundwater levels may be obtained from production wells in which the level has stabilised after a period of non-pumping, or from specifically constructed observation wells or piezometres. It is important to know the interval over which the aquifer is open to the observation well in order to avoid interpreting the water levels from two distinct aquifers as though they represented only one aquifer (Figure 1).

Figure 1. Water levels in multiple aquifer system.

Water levels, which should be reported in terms of some