By ERWIN CHLANDA
A study by the National Centre for Groundwater Research and Training describes the likely impact of the Rocky Hill vineyard on the borefield from which Alice Springs will be drawing its water in about 30 years’ time.
It says: “There is presently no evidence of contaminants derived from irrigation activities – either fertilisers, pesticides or herbicides – in the groundwater beneath the vineyard.
“This may be due to long travel times for water through the unsaturated zone.”
But “because concentrations of chloride, nitrate and other salts are naturally high in arid zone soils of central Australia” the report raises the possibilities that in the long term, “irrigation drainage will leach these salts into the underlying groundwater.”
This is not described as a major problem.
Proposed future pumping from the adjoining Power Water Corporation borefield on “could potentially draw groundwater from beneath the irrigation areas into the water supply”.
The paper describes the movement of underground water, its speed, the recharge and the possible impact both on the quality of the town’s future water and on the horticultural venture of the Hayes pastoral family, on the Undoolya Rocky Hill Agricultural Block, about 50 kilometres south-east of Alice Springs.
The public water supply of Alice Springs is currently largely sourced from the Roe Creek borefield, about 15 kilometres south‐southwest of the town, in the north‐eastern section of the Amadeus Basin, says the report.
“However, at current extraction rates, water levels at Roe Creek are expected to decline beyond economical pumping depths by about 2050,” says the report.
Rocky Hill has intermittently grown fodder crops “under centre pivot irrigation since the 1970s, with intensification of irrigation since 2002 when grapes were planted in the southeast of the block.
“Currently, there are about 60 hectares of irrigated vineyards at this site.
“Proposals have been developed for expansion of onions onto areas of Undoolya Pastoral Lease, immediately south of the current vineyard development, and a water licence to facilitate this development has been granted.
“The development of irrigated agriculture will inevitably lead to increased rates of groundwater recharge, and there is potential for this recharge to impact groundwater quality.”
Drainage water beneath irrigated viticulture “moves slowly down through the soil profile, pushing the pre‐existing soil water ahead of it.
“After a period of time, this pre‐existing soil water begins to recharge the underlying aquifer, which is at 50 to 55 meters in depth.
“Based on comparison of soil moisture profiles beneath the vineyards and adjacent native vegetation, and assuming that irrigation practices do not change, we estimate that this water will reach the aquifer within the next five to 10 years,” says the report.
“When this occurs, leaching of salts that were originally stored within the unsaturated soil profiles into groundwater will result in salt fluxes … into the underlying aquifer.
“Multiplying by an approximate area under grapes of 0.6 of a square kilometre gives 900 tonnes per year of salt.
“Assuming that irrigation at the site continues, then this salt flux will continue for a period of 50 to 80 years, by which time the pre‐existing soil water will have been completely flushed from the profile.
“The flux of nitrate will also increase [but] this is unlikely to result in nitrate concentrations above safe drinking water levels.”
The report says it has not specifically examined drainage rates or unsaturated zone travel times under onions or other crops, or the effect of solute fluxes beneath irrigation on the water quality of the proposed Rocky Hill borefield.
Under current conditions, due to the proximity of the irrigation bores and in the absence of any pumping from a PWC borefield, most of the recharge from beneath the vineyard will be extracted by the irrigation bores operated by the vineyard: “Simple calculations suggest that this might lead to an increase in total dissolved salt in pumping water … with potentially significant implications for vineyard management.
“However, it is likely that flow lines would change with the commencement of pumping from a PWC borefield , or if the locations of irrigation bores were to change.
“In particular, groundwater beneath the agricultural block is likely to be within the capture zone of” the proposed borefield.
“However, groundwater which is derived from beneath the agricultural block will be diluted by groundwater from other areas, and so it is possible that salt fluxes beneath the irrigation areas may have little effect on concentrations of groundwater pumped by such a borefield.
“A groundwater model of the Roe Creek and Rocky Hill – Ooraminna region is currently being developed, and this groundwater model should be used to calculate groundwater travel times to the borefield and the extent to which groundwater from beneath irrigation areas are diluted by groundwater from other parts of the aquifer.
“Such simulations could use a range of hypothetical bore locations and pumping rates.
“This report has also not specifically considered the consequences of future expansions of irrigation on the potential water quality of the production borefield.
“As a starting point, it is reasonable to assume that solute fluxes to groundwater will be similar to those beneath the current vineyard development.
“Thus, for example, if the area or volume of irrigation were to triple (in line with the recent increase in allocation), then it might be expected that the salt flux would also triple.
“If the groundwater model predicts that these solute fluxes are likely to have an adverse impact on the quality of groundwater sourced by the borefield, then additional studies to specifically examine drainage rates and travel times beneath other crops would be warranted.”
We have asked the manager of the horticultural venture, Ritchie Hayes, for comment.