Physical and Chemical Techniques for Discharge Studies - Part 1

By CSIRO PUBLISHING

Groundwater discharge is associated with salinity and pollution problems. The widespread presence of millions of saline lakes in North America, Africa and Australia, shows that across the geological record, most salinity and desertification problems have been caused by saline groundwater discharge. In recent times, dryland salinity has spread widely in southern Australia, resulting in the loss of more than 50% of the fresh streams in Western Australia and causing major salinity problems in the Murray River in South Australia.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Jan 1, 1996
• ISBN: 9780643106024

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1   INTRODUCTION

Groundwater discharge studies are not given the same importance as those of groundwater recharge. This may be due to the fact that recharge has always been associated with groundwater resources assessment, development and utilisation, while groundwater discharge is associated with salinity and pollution problems that have not been given sufficient attention except in the last twenty years.

The widespread presence of millions of saline lakes in North America, Africa and Australia shows that, across the geological record, most salinity and desertification problems have been caused by saline groundwater discharge. In recent times, dryland salinity has spread widely in southern Australia, resulting in the loss of more than 50% of the fresh streams in Western Australia and causing major salinity problems in the Murray River in South Australia.

The aims of this chapter are:

To define the discharge mechanisms that occur in nature

Explain how and where discharge takes place and what the effects of soil, water table and vegetation on discharge are

To introduce the mechanisms of biological discharge (plants, etc.) and the critical role it has in controlling groundwater recharge through its discharge mechanism (evapotranspiration)

To present the various physical, chemical and isotope techniques used for determining discharge rates

To discuss the various options available for enhancing groundwater discharge.

Groundwater Discharge

Groundwater discharge is the removal of water from the saturated zone of a drainage basin across the water table surface, together with the associated flow toward the water table within the saturated zone. In discharge areas the groundwater flow direction is upward and hydraulic head in the aquifer systems increases with depth. It may discharge as a spring, seep, or baseflow, or by evaporation and transpiration. The discharge rate depends on the size of the recharge area, the rate of rainfall or other water accretion in the area, and the transmissivity of the aquifer system or other water-conducting material.

Discharge areas are usually located in topographic lows, e.g. valleys, depressions and downstream ends of catchments and wide-scaled basins. The location is in principle controlled by the gravity-driven groundwater flow systems which are directly related to the groundwater basin’s topography. In a landscape of undulating topography, several discharge regions at different elevation levels and scale can exist, fed by local or regional flow systems (Fig. 1). Geologic structures such as faults, dykes or structural highs can impede or enhance groundwater flow and affect the manifestation of discharge areas (Salama et al. 1992).

Figure 1. Areas of point and diffuse discharge in relation to topography, geology and groundwater flow systems.

Two principal types of discharge can be differentiated and recognised in the field: point (focal) discharge and diffuse discharge. Point discharge is clearly defined and limited in its areal extent. Ascending groundwater is channelled or focused to its region of discharge, whereas diffuse discharge describes the continuous, spatially distributed discharge of water to the atmosphere resulting from upward groundwater flow from the water table to the soil surface.

Point discharge

Springs and seeps

Springs or seeps are the most conspicuous form of natural return of groundwater to the surface. They can be permanent or ephemeral and their discharge rate may show little change or may vary with time from an insignificant perceptible seepage to more than 90 m³ s−1 (Meinzer 1923). Springs and seeps occur where down-gradient sections of confined aquifers are exposed to the surface, e.g. at mountain sides, or where shallow water tables of unconfined aquifers reach the surface at the base of long slopes. Springs also form where geological discontinuities present hydraulic barriers and channel groundwater upward, allowing water to reach the surface if the potentiometric surface in the aquifer is sufficiently high. Fractured rock and fissures can fill with rainwater, which then flows through the fissure system to form springs at lower points. Rock debris and piles at the base of hills or mountains can temporarily store water in periods of rainfall and run-off and gradually release the water via springs or seeps along their periphery. Depression springs form as local discharge zones in topographic depression; contact springs result from lower hydraulic conductivity layers (clays at the bottom of sand dunes); and fault and fracture springs are created by geologic structures. A typical example of point discharge is the mud mounds created by the groundwater discharge in the Great Artesian basin (Williams and Holmes 1978). Another example is the sandhill seeps in the wheat belt of Western Australia, which occur at the bottom of sandplains underlain by clayey layers (Nulsen 1985; George and Frantom 1991).

Streams and lakes

Groundwater discharge into a stream (effluent stream) takes place when the water table lies above the stream surface. The amount of groundwater discharge is directly proportional to the hydraulic gradient towards the stream. Modification of the native vegetation, e.g. by deforestation has a pronounced effect on groundwater discharge to streams. Clearance of native forests in recharge areas is common practice in South and Western Australia (Williamson 1990). This causes the water table to rise and the hydraulic gradient and the seepage velocity to increase.

Lakes may be classified hydrogeologically on the basis of domination of the annual hydrologic budget by surface water or groundwater. Surface-dominated drainage lakes have inflow and outflow streams, while seepage lakes are groundwater-dominated by discharge (Fetter 1980). In most cases, groundwater seepage into lakes is highest