Key Features

Major Ion Chemistry

Cations (such as calcium, magnesium, sodium and potassium) and anions (such as chloride, bicarbonate, sulfate and bromide) have been used as tracers to determine groundwater input to a stream during high flow and low flow periods. Groundwater can have a chemistry that is distinctly different to the connected stream, and these characteristics can be used as indicators of groundwater discharge. Solutes that are present in groundwater are derived from two main sources: (i) input from rainfall, which have their origin from both marine salts and continental dust, and (ii) acquisition during weathering and water-rock interactions. Processes that affect hydrochemistry include (i) acid-base reactions, (ii) precipitation and dissolution of minerals, (iii) sorption and ion exchange, (iv) oxidation-reduction reactions, (v) biodegradation and (vi) dissolution and exsolution of gases. Hydrochemistry can be interpreted to understand the key processes that have occurred during the movement of water through aquifers and streams.

Representative samples of the stream and the groundwater system, as well as possibly other inputs such as rainfall, need to be taken. Protocols are available for the appropriate collection, field preparation and storage of these water samples (Table 1). Major anions (chloride, bromide and sulphate) can be determined by ion chromatography and major cations (calcium, magnesium, sodium and potassium) by atomic absorption spectrophotometry (AAS) or by inductively coupled plasma - atomic emission spectrometry (ICP-AES). These are routinely undertaken in most analytical laboratories. The accuracy must be demonstrated both by the use of appropriate standards and also by use of the ionic balance to check electrical neutrality.

Major ions data may be presented in graphical format, of which the most useful plots are the trilinear diagram that show the total major anion or cation composition on separate or combination (Piper) diagrams (Hem 1989). These diagrams have the advantage for tracer work of showing a large number of analyses in one plot to define distinct populations or trends. The relationships and evolution between dominant compositions (eg Ca-HCO3 to Na-Cl) usually indicate trends along flow paths or mixing between water bodies. Other types of diagrams include the mixing plot (Lawrence et al. 1976), Shoeller type diagrams and the Durov diagram (Howard and Lloyd 1983; Petalas and Diamantis 1999).

Table 1: Australian Standards related to water sampling (http://www.standards.com.au)
Australian Standard Title Content
AS 4276.1-1995 Water microbiology - General information and procedures information on the microbiological examination of water
AS/NZS 5667.1:1998 Water quality - Sampling - Guidance on the design of sampling programs, sampling techniques and the preservation and handling of samples general principles to be applied in the design of sampling programs, general guidance on sampling techniques and guidance on the procedures to be taken to preserve and transport samples for the physical, chemical and radiological analysis of waters and wastewaters, including bottom sediment and sludges, for the purposes of process control, quality characterization, identification of sources of pollution, compliance with water quality guidelines or standards, and other specific reasons.
AS/NZS 5667.10:1998 Water quality - Sampling - Guidance on sampling of waste waters detailed guidance on the design of sampling programs, sampling techniques and the handling and preservation of samples of waste water. It is identical with and has been reproduced from ISO 5667-10:1992.
AS/NZS 5667.11:1998 Water quality - Sampling - Guidance on sampling of groundwaters part of ISO 5667 that provides guidance on the design of sampling programmes, sampling techniques and the handling of water samples taken from groundwater for physical, chemical and microbiological assessment.
AS/NZS 5667.12:1999 Water quality - Sampling - Guidance on sampling of bottom sediments part of ISO 5667 that provides guidance on the sampling of sedimentary materials from inland rivers and streams; lakes and similar standing bodies; and estuarine and harbour areas.
AS/NZS 5667.4:1998 Water quality - Sampling - Guidance on sampling from lakes, natural and man-made, detailed guidance on the design of sampling programs, sampling techniques and the handling and preservation of samples of water from natural and man-made lakes. It is technically equivalent to and has been reproduced from ISO 5667-4:1987.
AS/NZS 5667.6:1998 Water quality - Sampling - Guidance on sampling of rivers and streams part of ISO 5667 that sets out the principles to be applied to the design of sampling programmes, sampling techniques and the handling of water samples from rivers and streams for physical, chemical and microbiological assessment.

References