Mining & inland waters
Not as a first option, and the EPA says so directly. Its stated position is that disposal of excess water to waterways should only be undertaken after other potential uses have been maximised — including mitigating environmental impacts, fit for purpose on-site activities, meeting the demands of other water users, and aquifer reinjection. So the first question a referral has to answer is not whether the receiving creek can carry the flow. It is why the water is going to the creek at all, and that has to be evidenced before the discharge design matters.
The order of preference, in the EPA’s own words
Dewatering is an essential component of many mining operations where mining occurs below the water table, and the EPA says so plainly. What it does not accept is discharge as the default destination for the surplus. The guideline names four things to be maximised first: mitigating environmental impacts, fit for purpose on-site activities, meeting the demands of other water users, and aquifer reinjection.
A submission that opens with the receiving environment and the licence limits has skipped the part that was asked for. The argument the EPA is looking for is the water balance that shows how much surplus is genuinely left once each of those four has been taken as far as it goes. The discharge design is what follows after the number stops shrinking — not the thing that stands in for the analysis.
The impact is the regime, not only the chemistry
The named risk is hydrological before it is chemical. Excess water not used through the mining operation is often discharged to local waterways, which can alter hydrological regimes with consequent changes to ecology and potential destabilisation and erosion of banks. The EPA separately lists, among activities with the potential to impact inland waters, the disposal of mine dewater to surface water systems where this may significantly alter the hydrology of the streams, with consequent impacts to aquatic or riparian ecosystems.
The quality list is worth reading closely as well, because it is not only metals. The EPA names dewatering discharges where the water quality — temperature, heavy metals, carbonates — is a concern. Temperature and carbonate chemistry are the two most often missing from a baseline programme that was scoped around a metals suite, and they are the two hardest to add retrospectively, because the pre-discharge condition they would have been compared against no longer exists.
Why a continuous discharge into an occasional creek is the hard case
The EPA’s definition of a waterway expressly includes systems that flow permanently, for part of the year, or occasionally. A great deal of Western Australian inland drainage is the third kind. A dewatering discharge that runs at a steady rate turns a channel that flowed for days in a year into one that flows every day of it.
Every water quality number in that scenario can sit inside its limit and the hydrological regime has still been changed fundamentally — which is the first half of the environmental objective, not a secondary consideration to it. Riparian vegetation responds to permanent water. Banks shaped by short, high flows behave differently under sustained low ones. The ecology that establishes is not the ecology that was there, and at closure the discharge stops and that new ecology loses its water source.
An assessment that anticipates this describes the regime change and its consequences directly, including what happens when the discharge ceases. One that does not leaves the reviewer to infer it from a compliance table, and the query comes back.
What the evidence has to cover
From the guideline’s information list, applied to a discharge: characterise the receiving system — hydrology, water quality, catchment boundaries, geology and connectivity, locally and regionally; describe how excess water is to be disposed of and how that might impact the environment; characterise the waste generated and the pathways for potential contamination, quantified through models where necessary; and predict the changes to the surface water regime that result.
Then follow the water. The guideline asks for the potential consequences of any hydrological or water quality change on downstream waters such as estuaries or the marine environment — so a discharge assessment that stops at the property boundary has stopped early. It also asks for significance evaluated across direct, indirect and cumulative impacts in a local and regional context, cumulative being the one that catches an operation in a catchment where neighbours are doing the same thing.
And then the arrangements: monitoring, mitigation, management, closure and rehabilitation, the predicted outcome against the environmental objective, whether a significant residual impact is likely, and the adaptive management or contingency planning that applies if the predictions are wrong.
Part IV is not the only approval
An EPA assessment under Part IV of the Environmental Protection Act 1986 is the impact assessment. A discharge from a prescribed premises is separately regulated under Part V, where a works approval is required before construction and a licence or registration is required before an emission or discharge is caused. The two run on different timetables and ask different questions of the same water.
They do draw on the same evidence, though. Receiving-environment characterisation, the water balance and the discharge quality prediction are inputs to both, and building them once to a standard that satisfies the harder reader is cheaper than building them twice to two different ones.
When the work is worth doing
The surplus volume is an output of the mine plan and the dewatering schedule, so it moves whenever those move. Building the surface water case late means building it against a number that is still changing, and the version that reaches the regulator is the one that had the least time.
The unglamorous alternative: a water balance early, receiving-environment baseline started before there is a discharge left to characterise it against, and a discharge case written from a position where the four alternatives were genuinely tested rather than retrospectively justified.