Lower River Murray Reclaimed Irrigation Area (LMRIA)
The Lower Murray Reclaimed Irrigation Area (LMRIA) is approximately 5,200 hectares of flood irrigated agricultural land protected by a levee bank system on the former floodplain of the River Murray in South Australia.
The LMRIA is located between the townships of Mannum and Wellington and comprises 24 individual irrigation areas. Historically, dairy farming was the predominant land use with a smaller area for beef cattle, fodder production and lifestyle farming.
The irrigation bays are typically 1.0–1.5 m below the normal river pool level (+0.75 m AHD), enabling gravity fed flood irrigation. The excess runoff from irrigation, as well as highland groundwater seepages, are returned to the River Murray via a drainage network and pumping system.
Due to very low inflows from the Murray–Darling Basin from 2007–09, water levels in the Lower Murray River (below Lock 1) fell from a normal pool level of +0.75 m AHD to a low of –1.05 m AHD in April 2009.
The low water levels and restricted water allocations during this hydrological drought period meant that most of the LMRIA was not irrigated and led to a drop in the shallow water table of 1.5–2 m from pre-drought levels. The heavy clay soils subsequently salinised, dried and cracked causing major damage to the rehabilitated irrigation bays and associated infrastructure, and major socio-economic impacts.
When irrigation recommenced in 2010–11, orange coloured water in the drainage channels was noted by Jervois Irrigation Area landholders. Screening by the EPA highlighted the presence of acid water in this and 13 other drainage channels in the LMRIA region.
Further investigations by the EPA, in conjunction with the CSIRO, found that acid sulfate soils are prevalent in the LMRIA region. It was determined that the falling groundwater levels during the drought allowed these soils to be exposed and oxidised, which produced large quantities of sulfuric acid in the soil matrix.
When irrigation takes place on an LMRIA irrigation bay, this stored acidity in the soil enters the groundwater and is eventually flushed into the drainage channels. This drainage water is then discharged to the River Murray. The discharge of the water is necessary to maintain agricultural practices in this area.
The acidic drainage water entering the River Murray poses a potential risk to the water quality and environmental values. The EPA, Murray–Darling Basin Authority (MDBA) and DEWNR are undertaking a project to examine the nature and risks of acid drainage originating from the LMRIA, through monitoring and research. This work will inform development of management options to both protect the environmental values in the River Murray and farm productivity in the region.
Management and remediation methods
The EPA is working with other government departments, the MDBA, SA Water, the local community and landholders to trial possible remediation options to prevent the discharge of acidic drainage water to the River Murray.
Option 1: Large-scale flood irrigation
A trial site was set up on an irrigation bay at Long Flat irrigation area to test whether large-scale flood irrigation would be beneficial in 'flushing' the acidity out of the soil profile when flows were very high and sufficient to dilute the acidity. Groundwater and surface drainage water was intensively monitored over a series of irrigation events.
The data from this trial illustrated that irrigation, while providing positive benefits for soils and pasture, only very slowly decreases acidity present in the deeper subsoil and groundwater and results in acid export to the drainage channels and river. The results of this trial can be viewed in the preliminary report.
Option 2: Surface limestone spreading followed by flood irrigation
This trial involved spreading approximately 60 tonne of superfine limestone over three hectares at the Long Flat trial site. Two irrigation cycles (over 4–5 weeks) were performed to try to 'wash' the limestone into the soil profile through the previously fragmented and cracked soil structure. Sampling and monitoring protocols were replicated, as in previous irrigation trials.
Intensive monitoring illustrated that there was a small increase in pH and a reduction in retained acidity and available acidity, especially in the top 1 m of the soil profile. However, it is expected that it may take several months before the effect of the lime on the soils is noticeable, and improvement in soil acidity is seen. Monitoring at the sites is conducted on a monthly basis, and will be reported later in 2013.
Option 3: In situ salt drain acidity neutralisation
This trial aims to treat the acidic water in the drainage channels by introducing a neutralising reagent prior to discharge to the River Murray. The reagent is a 5–10wt% hydrated lime slurry and is applied to the drain using a flexible and mobile truck-mounted dosing system, including a monitor cannon and approximately 100 m of hosing.
Continuous water quality monitoring was conducted along the length of the salt drain, and before and after discharge to guide and assess the success of the salt drain treatment. Results suggest that the acidic drainage is able to be successfully treated using this method. A neutral pH was achieved, and acidity and associated soluble metals are eliminated before discharge to the River Murray.
This is expensive, and does not treat the problem at the source (in the soils under the irrigation bays), meaning treatment would have to be in place at each LMRIA discharge for a long time period of time.
Option 4: Subsoil remediation using a modified mole plough
This trial aims to introduce a neutralising material (lime slurry) to the subsoil by using a ploughing technique commonly practised in the LMRIA area. The mole plough was originally used in the LMRIA to form gravel-lined channels to assist drainage in clay soils.
In this trial, the lime slurry was used in place of the gravel in the mole drain. This method delivered the neutralising material directly to the subsoil and the zone of acidity (1 m below ground), without disturbing the pasture or topsoil. By creating pockets in the soil profile where the acidic groundwater and soil can be neutralised, it is hoped that the process of remediation can begin in these soils.
The trial has been conducted in the Mobilong irrigation area, a retired site owned by SA Water. Two paddocks were used, one as the trial site and one as a control. The mole ploughing was undertaken to a depth of 700 mm and ran along several parallel lines along the length of the paddock approximately 4–5 m apart.
The ploughing was followed by a series of irrigation events, which helped to spread the lime in the subsoils. Regular monitoring was conducted to ascertain change in acidity in the acidic soil zone. The first trial of this method was undertaken in October 2012 and the second in December 2012.