Aquaculture Groundwater Resource Atlas - Groundwater chemistry

General

Although the groundwater salinity is highly variable across the State, most groundwaters in WA have a chemical composition similar to that of seawater and are generally suitable for growing estuarine and marine fish. However, fish are sensitive to some chemical parameters in very low concentrations.

Generally groundwater has a low concentration of dissolved oxygen and may contain toxic substances such as hydrogen sulphide, and is therefore unsuitable for pumping directly into fish ponds without aeration. Groundwater with a pH of less than 4.5 may also contain high concentrations of metals such as aluminium and iron that can retard fish growth. Acidic groundwater should be treated to reduce its acidity before aeration to reduce concentrations of metals toxic to fish.

At low salinity levels, less than 1 ppt TDS (Total Dissolved Solids), groundwaters are dominated by calcium, magnesium, sodium, and bicarbonate and chloride ions, with minor proportions of potassium and sulphate. With increasing salinity, the proportions of sodium and chloride increase towards seawater composition. These ions are known as major ions, are generally included in chemical analyses of groundwaters.

Minor ionic constituents are iron, managnese, nitrate, silica, boron and fluoride. Iron occurs in concentrations up to 0.02 ppt, and often requires treatment by aeration to precipitate iron as oxide. Manganese is rare but can occur in concentrations as low as 0.001 ppt which similarly requires precipitation by oxidation before use.

Nitrate values in potable groundwater range up from background of less than 0.001 ppt in the Southwest up to values exceeding 0.045 ppt in inland WA, which is the upper limit recommended for drinking water. Silica values range up to around 0.1 ppt in low salinity groundwaters, especially in the north of WA. Boron and fluoride are generally less than 0.002 ppt in potable groundwaters.

Heavy metals occur at very low concentrations, but may be elevated in areas of mineralisation.

Groundwaters in WA generally range from pH 6 to pH 8, but can be acid (pH 3.5) to slightly alkaline (pH 8.5). Acid groundwater occurs in deep aquifers in the Perth Basin, and in hypersaline groundwater in the goldfields. Acid groundwater may also occur as a result of oxidation of sulphides in mining areas where the groundwater regime has been altered to expose minerals.

Radon gas occurs naturally in all groundwater, and is removed by aeration. Concentrations are relatively low in porous sediments and high in fractured rock aquifers.

Contaminants may leach to groundwater from human activities on the earth's surface. A range of contaminants occur around industrial waste disposal sites, and landfills. Leaching of fertiliser, principally nitrate and sulphate, with minor phosphate occurs beneath horticultural crops. Pesticides may occur in shallow groundwater, but have not been identified at depth in the Perth Basin aquifers (Hirschberg and Appleyard, 1996).

Information on groundwater chemistry from borehole samples is held in the Water and Rivers Commission WIN database.

What is the groundwater chemistry in your area?

To find out more you may take the following steps:

1. Identify the aquifer you are interested in targeting from the aquifer map and text available on this Web page. Contact the Water and Rivers Commission's Information Officer with the aquifer name and site location details on (08) 9278 0580 or waterinfo@wrc.wa.gov.au requesting groundwater chemistry data. Specific information may be requested on www.wrc.wa.gov.au/waterinf/wric/DataRequestPage.asp.
2. Identify your neighbours' bores which may be pumping from your chosen aquifer. Speak to the owner/operators. Have they representative groundwater chemistry data? If not you could request permission to sample. Look in the Yellow Pages under "Analyst" or " Environmental Consultants". Ask for sampling equipment and advice. Take groundwater samples from bores which have been pumping for a few hours or more to obtain representative chemistry data for the aquifer and have these couriered to the laboratory.
3. If local bores have favourable yields and groundwater chemistry you may now seek specific advice from your Regional Fisheries and Water & Rivers Commission offices. This is important before committing yourself to the expense of sinking and constructing a bore.

Sinking a bore

You are advised to seek the advice of an experienced consulting hydrogeologist and/or driller before deciding on the location, depth and construction details of your bore. A well designed and constructed bore will last many years and may have cheaper maintenance and quicker pay back than repeatedly drilled bores. Issues to consider are:

• Maximum and minimum rates of flow
• Pumping costs
• Life time of the bore
• Standby bore

Groundwater licensing

Groundwater abstraction in proclaimed groundwater areas requires a groundwater abstraction licence from the Water and Rivers Commission. These cover most of Western Australia, with the main exception being the Wheatbelt, most of which is exempt from licensing.

Groundwater areas in the Perth Basin have an allocation limit set for each sub-area. The current licensing situation for particular groundwater areas can be obtained from the Water & Rivers Commission's regional offices. Link to allocation or regional offices on the Commission's web page, www.wrc.wa.gov.au/region/index.html

Bore Yields

Bore yields depend on a number of factors, the capacity of the aquifer, the construction of the bore, and the type of pump used.

In the text descriptions, yields from aquifers are classified as high, medium or low.

Bore yields reported by the Water and Rivers Commission's WIN database may not reflect the potential of the aquifer. For instance, many farm bores are equipped with windpumps, and are therefore not constructed to maximise the yield. Bores may only penetrate the top of the aquifer, and may have slotted casing, instead of a bore screen. Generally, the longer the screen, the higher the yield, if there is sufficient thickness of aquifer. You may contact a Water & Rivers Commission hydrogeologist for advice on groundwater@wrc.wa.gov.au.