RECENT data shows the Ord River once debouched into the Keep River, and not into the Cambridge Gulf near Wyndham which is its present course.
Hydro-geologist Dr Richard George says a 2011-12 airborne electro-magnetic survey confirmed the presence of a paleochannel connecting the two rivers.
This is now the main aquifer below the new Goomig farmlands to be developed with the Ord Irrigation Expansion Scheme.
Dr George, who is a principal research scientist with the Department of Agriculture, says a team of about 10 people worked for 350 days in the field to complete five technical reports comprising the environmental assessment for the scheme.
He describes these as hydro geology; soils investigations; geochemistry; aquifer testing; and Keep River water quality flows and assessment, including runoff modelling from agricultural lands.
âEach one of those reports has new findings,â he says.
âWe found the paleochannel, documented its hydraulics and permeability, [and] proved that the hydraulics of that aquifer was the same as in Stage I [the original 1960s irrigation scheme].
âThe geochemistry work proved that if we are ever required to go to aquifer management we could manage all levels, manage the discharge from any of those pumping schemes into either the Keep River or the supply channel.
âThe increase in tropical rainfall across the last 15 years or so has led to very significant rises in groundwater levels across these areas and weâve shown that thatâs not related to agriculture.
âThat north-west part of WA has gone through a big increase in rainfallâup to 30 per cent or more in some yearsâand the groundwater systems and the river systems have responded.
âThe Keep River now permanently flows.â
He says the river is also more saline as increased rainfall causes the water table to rise.
The downstream river study was the beginning of a continuing process to set the triggers and environmental conditions around which agriculture will impact on the river, and to avoid negative impact on protected species.
âThere had been very little if any hydrochemistry done on that river, though weâve obviously got a lot of that data now.â
âWe found the river has some large nitrogen and phosphorous loads which are pre-agriculture or natural.
âThatâs partially a function of pastoralism but itâs also a function of the big wet seasons that have occurred in our monitoring period, we get massive decomposition of all the flood organics.â
The Resource Management Technical reports, numbered 366-370, can be found here.