Geochemical and Isotopic Constraints - Ifchemania

Groundwater Flow Systems and Salinity in the Valleys around Jamestown, South Australia: Geochemical and Isotopic Constraints

Richard G Cresswell and Andrew L Herczeg

CSIRO Land and Water Technical Report No. 30/04 June 2004

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Cover Photograph: Salt-bush has been planted in the open valleys south of Jamestown to help lower shallow watertables causing dryland salinity. Photographer: Richard Cresswell © 2004 CSIRO

JAMESTOWN Groundwater Flow Systems and Salinity in the Valleys around Jamestown, South Australia: Geochemical and Isotopic Constraints Richard G. Cresswell and Andrew L. Herczeg

CSIRO Land and Water Technical Report 30/04 / BRS Report ZZZ

May 2004

This report builds on the South Australian Salinity Mapping and Management Support Project funded by the National Action Plan for Salinity and Water Quality. The National Action Plan for Salinity and Water Quality is a joint initiative between the State and Commonwealth Governments

SA-SMMSP Jamestown Hydrogeochemistry Report

Addresses and affiliations of authors

Andrew L. Herczeg Cooperative Research Centre for Landscape Environments and Mineral Exploration c/o CSIRO Land and Water Private mail Bag 2 Glen Osmond SA

Richard G. Cresswell Bureau of Rural Sciences GPO Box 858 Canberra ACT 2601 Australia Present address: Cooperative Research Centre for Landscape Environments and Mineral Exploration c/o CSIRO Land and Water Long Pocket Laboratories 120 Meiers Rd Indooroopilly QLD 4068 Australia

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SA-SMMSP Jamestown Hydrogeochemistry Report Executive Summary This report aims to provide some geochemical constraints for the groundwater systems around Jamestown in the Northern Agricultural District of South Australia, and hence provide further support for models of dryland salinity in this region. These groundwater systems must be placed in the context of the materials comprising the landscape, and a companion report (Wilford, 2004) examines the regolith architecture of the region, with emphasis on the use of airborne geophysics (AG) to help our understanding in three dimensions. That report also examines some of the ramifications of the nature of the landscape on salinity in the area. Local hydrogeological investigations have been summarised in Henschke, et al. (2004) and are combined with the inferences of the airborne geophysics as the basis for examining groundwater connectivity in the sub-surface. Specific objectives of this project were: 1. Evaluate the groundwater pathways as suggested by airborne geophysics; 2. Assess recharge mechanisms for the groundwater systems based on chemical constraints; 3. Determine the connectivity between the surface waters and the deeper groundwaters; 4. Determine the source(s) of salt in the groundwater. The environmental isotopic composition of the groundwaters sampled from the observation bores show minimal divergence from the meteoric water line suggesting that diffuse recharge is the dominant infiltration process for the region. The highest 14 C contents were measured form the flank of the central, Bundaleer Valley, sited in the colluvial aprons along the margins of the valleys. This suggest rapid, recent recharge is occurring through these regions. The deep bore in this area penetrates the alluvial sequence, sampling groundwaters at the base of the alluvials. This is the freshest bore sampled in the region, but has a relatively low 14C concentration of 46% modern carbon (pMC) implying older waters (up to 5,000 years). We surmise that there is a significant component of recharge from higher in the valleys contributing to the groundwaters at this site bringing older waters to mix with the young recharge from the valley flanks. Lithology contrasts between the colluvial flank deposits and the valley alluvials may also limit exchange of waters between these two sources. Bores in the eastern, BelalieValley also show low 14C (