Hydrogeochemistry of the upper Hunter River Valley, New South Wales

The chemistry of groundwater in the regional recharge zones of Triassic and Carboniferous rocks in the upper Hunter River valley of New South Wales is strongly influenced by silicate and carbonate dissolution/precipitation reactions, ion exchange and the dispersion of aerosols in infiltrated rainfall. The Wollombi Coal Measures and Jerrys Plains Subgroup of the Wittingham Coal Measures west of the Muswellbrook Anticline constitute the regional groundwater transmission zones, and the processes having the greatest influence on the chemistry of their water are dissolution/precipitation reactions and oxidation of coals. The semi-confined aquifers of the Greta Coal Measures, Maitland Group, Dalwood Group, and Wittingham Coal Measures in the eastern and southern parts of the valley discharge into unconfined sand and gravel aquifers of the Hunter River floodplain. These Permian rocks are the source of the most saline water in the valley, and the chemistry of their groundwater is largely determined by oxidation of sulphides and molecular diffusion of connate marine salts, a legacy of periodic immersion by Permian ocean water. Disequilibrium indices for calcite, dolomite and dawsonite indicate that these carbonates are being precipitated today in the groundwater of the Central Lowlands provinces; they are being dissolved in the southern and western groundwater recharge zones and are in equilibrium with water of the northern recharge zone. The iron carbonates, siderite and ankerite, are a product of a palaeohydrochemical regime characterised by saline alkaline water rich in dissolved iron disseminated from gels originally accumulated in the Permian peat swamps, but these minerals are not being precipitated in modern upper Hunter River valley groundwater. The sulphate minerals, gypsum, thenardite and bloedite, occur extensively in salt efflorescences in the Permian rocks of the Central Lowlands, but their disequilibrium indices show that none of the minerals can be precipitated in the contemporary upper Hunter River valley groundwater by processes other than evaporative concentration. Models based on incongruent dissolution of feldspars allocate much of the upper Hunter River valley groundwater to the kaolinite stability field, which is consistent with the abundance of kaolinite as an authigenic mineral in the fractured rock aquifers. Silica and cations leached from the fractured rocks are accumulating in the groundwater sinks around the margins of the Hunter River floodplain, as indicated by the large proportion of groundwater in these areas which are in equilibrium with Ca-montmorillonite. Concentrations of C a 2 + , S i 0 2 and H C 0 3 ions in upper Hunter River valley groundwater approach log-normal distributions and these species are most highly identified with continental hydrochemical processes. In contrast, the four 'elements' constituting the bulk of solutes in ocean water, CI", N a + , SOj" and Mg2 + , are distributed in two modes: the low-concentration primary mode, representing the dissemination of these species from the continental solutes store, and the secondary high-concentration mode, reflecting diffusion and oxidation of marine inputs. On a province-wide scale, composition diagrams of solute behaviour identify the Wittingham Coal Measures to the east and south of Muswellbrook Anticline, the Greta Coal Measures, and the Maitland and Dalwood Groups as systems that can be approximated by simple linear mixing models between meteoric and oceanic water. Composition diagrams for the floodplain hydrochemical provinces show that the alluvial aquifers can be represented as mixing systems between Hunter River surface water and groundwater of the fractured-rock aquifers. Principal component analyses describe the chemical evolution of upper Hunter groundwater from the Permian marine transgression through to the present continental leaching regime for similar positions along flow lines in discharge zones, groundwater of the Greta C

Tags: AU-NSW, Bulletin, Earth Sciences, GA Publication, Published_External, geochemistry, hydrogeology
Organization: Australian Ocean Data Network