Townley, L.R., Turner, J.V., Barr, A.D., Trefry, M.G., Wright, K.D., Gailitis, V., Harris, C.J., and Johnston, C.D. (1993), Interaction between lakes, wetlands and aquifers, CSIRO Division of Water Resources, Consultancy Report 93/1, 469pp.

This Report presents the results of a three-year Project on the interaction between lakes, wetlands and unconfined aquifers. Apart from a scientific goal of achieving greater understanding of lake-aquifer interaction, the Project had three practical objectives relating to the identification of capture zones, management of water levels and the development of effective parameters for groundwater flow models in plan.

Identification of Capture Zones

This Report focuses on lakes, rather than on sumplands or damplands. The majority of lakes on the Swan Coastal Plain act as flow-through lakes which capture groundwater on their upgradient side and discharge lakewater on their downgradient side. Contaminants in groundwater are carried in the direction of groundwater flow, but also mix laterally within the aquifer and can be retarded by sorption or decay. We have developed two-dimensional models in vertical section, two-dimensional models in plan and three-dimensional models in order to study the shape of capture and release zones as a function of nearby aquifer flows and net groundwater recharge. The depth of a capture zone depends mostly on the length of a lake in the direction of average groundwater flow relative to the thickness of the aquifer. The depth of a capture zone also depends on aquifer anisotropy, the resistance of low conductivity bottom sediments, aquifer inflows and outflows, and recharge. The width of a capture zone in plan is roughly twice the width of the lake. The depth of a release zone is closely related to the depth of a capture zone. Groundwater seepage into and out of a flow-through lake is concentrated near the upgradient and downgradient edges of the lake. The shape of a lake's release zone can be identified in the field by taking water samples and analysing for isotopic and hydrogeochemical concentrations. We have studied the release zones of Nowergup Lake, Mariginiup Lake, Jandabup Lake and Thomsons Lake using isotopic and hydrogeochemical tracers. Isotopic and hydrogeochemical tracers have confirmed that outflow from Lake Pinjar becomes inflow to Nowergup Lake, a distance of 5.75 km downgradient. The most cost-effective way to learn about a lake's release zone and hence its groundwater flow regime is to install a nest of piezometers or a multi-level piezometer at the middle of the downgradient side of a lake. Measurements of piezometric heads upgradient and downgradient of a lake can in principle give information about the geometry of capture and release zones, but are not as conclusive as isotopic and hydrogeochemical data. The concentration of isotopes and chloride in lakewater and a lake's release zone can assist in the determination of a lake's water balance. Capture zone geometries vary seasonally as lake levels and surface areas fluctuate. Capture zones of lakes on the Swan Coastal Plain can be determined by regional scale modelling, coupled with results of idealised modelling of isolated lakes. Nitrate, phosphate, petroleum products and pesticides can all be carried by groundwater, but some are degraded or retarded, thus reducing their rate of movement through an aquifer. The capture zones of lakes have implications for landuse planning, in that it may be desirable to control some potentially polluting activities within the capture zones.

Management of Water Levels

Water levels in lakes on the Swan Coastal Plain fluctuate seasonally, and in some cases, lakes are dry at the end of summer. Lake levels can be effectively maintained by pumping relatively small volumes of groundwater into the lakes for a few months each year. Artificial water level maintenance can lead to an improvement in lake water quality. In order to minimise its impact on lakes, pumping for public or private water supply should be located as far away as possible, both in space and in time. Average groundwater and lake levels depend on long-term average recharge, whereas seasonal fluctuations depend on the deviations between fluctuating recharge and the long-term average. Long-term fluctuations in groundwater and lake levels depend on long-term fluctuations in recharge. Lake levels can fluctuate either more or less than nearby groundwater levels, depending on whether a lake is driven by surface water inflows or by groundwater inflows.

Effective Parameters for Models in Plan

Groundwater flow patterns near shallow lakes are fundamentally three-dimensional, but we have developed approximate methods for representing lakes in two-dimensional regional models of aquifer flow. We have developed guidelines for assigning large transmissivities to represent circular lakes in a one-layered model of a regional aquifer. We have also developed guidelines for assigning leakage coefficients to represent circular lakes in a two-layered model of a regional aquifer. Field data on the hydraulic conductivities of lake linings confirm that they are often low, but we have not related measured values to effective values needed to represent lakes in two-layered plan models.

Other Findings

In the process of reviewing simple methods for predicting the movement of phosphate fronts, we have discovered inconsistencies and developed a new method for predicting travel distance. We have reviewed the development of isotope balance equations for evaporating water bodies, and summarised previous literature in a concise unified framework. We have summarised the correct way of determining the angle between equipotentials and directions of flow in a vertically exaggerated cross-section through an anisotropic medium. We have developed the theory for a fully coupled groundwater and surface water model, which solves simultaneously for groundwater and surface water levels in a vertical section or in three dimensions. We have developed a method for combining measurements of all the components of a lake water balance to obtain estimates of the same components which are constrained to satisfy an exact water balance.

This report was scanned in June 2015, 22 years after completion of the Final Draft. The report is now available for download [21.95 MB].