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Investigating the impacts of groundwater on soil properties and pasture nutrition in irrigated agriculture, Pilbara region of Western Australia

Yeap, Simon G.H. (2014) Investigating the impacts of groundwater on soil properties and pasture nutrition in irrigated agriculture, Pilbara region of Western Australia. Honours thesis, Murdoch University.

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Dewatering of groundwater systems has become a common practice for iron ore mining in the Pilbara region of Western Australia. While the discharge of surplus water to local tributaries and re-injection into the aquifer are widely practiced, the re-use of this water for irrigated forages is an innovative solution. However, the chemistry of the groundwater and the impacts on soil properties from long-term application of groundwater need to be assessed.

Surplus water from the Marandoo iron ore mine is utilised to irrigate Rhodes grass (Chloris gayana) for hay production at the Hamersley Agricultural Project (HAP). After amendment with nutrients, the irrigation water was slightly alkaline (pH 8.0) and slightly brackish-sodic (total dissolved solids, TDS, of 580 mg/L) with Ca (61 mg/L), Mg (50 mg/L) and Na (43 mg/L) as the dominant cations and bicarbonate (270 mg/L) as the dominant anion. This study aims to identify the implications of irrigation with this water for pasture production and soil management.

Following the commencement of irrigation in October 2012, significant changes and trends in soil properties and leaf nutrient composition of C. gayana were examined over a 15 month period, based on a quarterly sampling program across 10 centre-pivot irrigation systems. Analysis initially showed that the continuation of current trends could result in: (1) increases in soil sodicity, since ESP levels had exceeded 5% at 0-10 cm and 7% at 20-30 cm, and (2) alkalinisation, such that the soil pH is predicted to reach ~8.2. However, subsequent analysis with pre-washed soil samples to remove soluble salts indicated that irrigation had not caused a measureable change in the ESP and hence no change in the sodicity of HAP soils.

Nonetheless, the geochemical model WEB-PHREEQ suggests the precipitation of carbonate, (hydr)oxide and phosphate (apatite) minerals of Ca, Mg, Fe and Mn could also impose a risk for immobilising nutrients applied from irrigation water, given suitable conditions for nucleation and crystal growth. Moreover, changes in the relative abundance of soil exchangeable cations may also adversely affect plant nutritional balance whereby exchangeable Mg2+ as a percentage of cation exchange capacity has significantly increased while the percentages of exchangeable Ca2+ and K+ have significantly decreased.

In the next 20 years, based on the estimated duration of the HAP, soil alkalinisation could emerge as a problem by suppressing the availability of various nutrients. Future monitoring and research, in conjunction with effective irrigation and soil management practice, will hence be imperative to ensure long-term sustainability of pasture production at the HAP, as well as for rehabilitation of soils after decommissioning.

Publication Type: Thesis (Honours)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Bell, Richard and Harper, Richard
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