Publication Date

12-2019

Series Number

404

Publisher

Department of Primary Industries and Regional Development

City

Perth

Abstract

The Myalup region, which incorporates the Myalup Irrigated Agriculture Precinct (MIAP), is located about 100 kilometres south of Perth between the southern end of Lake Clifton and the northern tip of the Leschenault Inlet. It covers 17 800 hectares, with about 2100ha being irrigated. The precinct produces mainly carrots, onions, potatoes and leafy vegetables. In 2016–17, the total value of agricultural production for the MIAP, including on-farm processing and packaging, was more than $120 million.

Irrigation water is mainly from the surficial Myalup aquifer, with limited abstraction from the underlying Leederville aquifer. Average annual rainfall is about 800 millimetres, but rainfall has been declining since the 1970s and has reduced by about 13% since 2000.

Salinity levels were increasing in some irrigated areas with ions, such as sulfate and chloride, reaching levels of concern to growers and regulators. There was also concerned about the saltwater intrusion risk into the superficial aquifer from salt lakes and the ocean to the west, and the source of increasing salinity in areas to the east.

There is a growing water demand in the MIAP as a result of horticultural relocation to the area from Perth and other areas. Diversifying existing sources of fresh food and vegetable production and securing the long-term sustainability of water supplies are essential to maintaining and growing food production. Salinisation of the superficial aquifer poses a risk to production and a potential constraint on the future development of agriculture in the area. As part of the Water for Food program funded through Royalties for Regions, DPIRD investigated the likely causes of salinisation in the MIAP.

Previous investigations showed there was considerable spatial and temporal variability of salinity in the superficial aquifer, with the origins of the salinity poorly understood.

To better understand the controls on salinity in the Myalup aquifer, licensee water quality data from 200 sites were collated in a database. The database analysis was complemented with two phases of groundwater sampling across the Myalup region, covering a broader range of water quality parameters than the licensee data.

The licensee and groundwater data showed there were multiple geochemical processes contributing to the increased salinity along the groundwater flow path in the Myalup aquifer. Salinity trends at sites with irrigated agriculture were statistically analysed using groundwater licensee data (140 sites out of 200) to determine the extent of increased salinity in the Myalup aquifer. Only 30 of the 140 sites showed increased salinity, while six sites showed decreased salinity. Within the sites showing an increase in salinity, oxidation of pyrite in acid sulfate soils was the dominant source of salinity at 19 sites (63% of 30 sites). In two localised areas (nine sites), recirculation was the main salinity process, and intrusion of saline groundwater was the main process in just one locality (two sites).

Number of Pages

193

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