• The nitrate MAV is a short-term value. It was set to protect bottle-fed infants against methaemoglobinaemia. There is no averaging, no percentile and no transgression allowance. A single result above the MAV is an exceedance and the response is immediate.
• Nitrate and nitrite are assessed together. Each is measured against its own MAV and the two ratios are summed. The sum must not exceed 1. Water can be below both individual MAVs and still fail.
• Two of the treatment options in this tool make nitrite. Ion exchange run past breakthrough, and biological denitrification with an inadequate carbon dose. The sum-of-ratios rule is the reason that matters.
• The monitoring engine reads the rules level and the source class and returns the applicable DWQAR 2026 rule, the frequency, and what escalates it. Enter the last twelve months of results as counts and it will also test the step-down conditions.
• The risk panel returns no verdict. It places the water against reference lines of three different kinds — legal, advisory and research — and states what each one is. Read the About panel section “What this evidence does and does not establish” before any of this goes into a document that reaches a community.

• The bore is not drinking today’s rain. It is drinking water that entered the ground years to decades ago, carrying the nitrogen that was leaching then. A change in land use today does not appear at the bore today. It appears over the residence time of the aquifer.
• Loading already in transit. If the nitrate in recharge entering the aquifer now is higher than the nitrate at the bore, then the bore is still rising and it will keep rising even if nothing in the catchment changes. That is the committed load. A supply can be compliant today and already be committed to an exceedance, and no treatment decision made on today’s number will be the right size.
• It runs both ways. An intensification and a reduction obey the same time constant. The scenarios below cover both, because a supply needs to know what a worsening catchment does to it as much as what an improving one does.
• Two separate questions, and conflating them is where the argument goes wrong. How much the loading has to change is set by the steady state, and it does not depend on the residence time at all. How long it takes to arrive is set by the residence time, and it does not depend on how much. The tool answers them in separate panels for that reason.
• The model is an idealisation and its assumptions are printed at the bottom of this tab. The mean residence time is rarely known better than a factor of two, and every time in the output scales with it.

• Resin choice is the whole design. A conventional strong base anion resin prefers sulphate to nitrate. A nitrate-selective resin (tributylamine or triethylamine functionality) reverses that preference at the cost of some capacity. On the same water the run lengths differ by an order of magnitude.
• Chromatographic peaking. On a sulphate-selective resin, nitrate breaks through first and is then displaced off the resin by the advancing sulphate front. The effluent nitrate rises above the feed concentration until sulphate breaks through. Against a short-term MAV that is an acute exceedance. On a nitrate-selective resin the preference is reversed, nitrate leaves last, and there is no peak.
• Partial treatment saves money. The treated water leaves the column at essentially zero nitrate, so you rarely need to treat all of it. Send a fraction through the resin, bypass the rest of the same bore around it, and recombine to hit your target. A smaller plant, less salt, less waste. The panel below tells you the smallest fraction that still meets the MAV.
• The product water is a different water. Chloride-form resin exchanges chloride for nitrate, sulphate and bicarbonate. Chloride rises, sulphate falls, alkalinity falls, pH falls. The chloride-to-sulphate mass ratio rises, and a rising CSMR is a recognised driver of lead and copper release from plumbing. This is how a nitrate fix creates a lead problem.
• The nitrogen goes into the brine. It is not destroyed. The kg N/day figure below is the number to take to whoever owns the disposal route.

• Nitrate is rejected less well than most ions. It is small, monovalent and weakly hydrated. Typical observed rejection is 85 to 95%, not the 98 to 99% that a membrane datasheet shows for sulphate and hardness. Sizing an RO plant on the datasheet salt rejection will undersize it for nitrate. The default here is 90% and it is editable in Settings.
• Partial treatment saves money. Permeate is near-zero nitrate, so you rarely treat all the flow. Send a fraction through the membranes, bypass the rest of the same bore around them, and recombine. Bypassing raw water also restores some hardness and alkalinity, which reduces the remineralisation duty. The panel below gives the smallest fraction that still meets the MAV.
• Permeate is aggressive. Near-zero alkalinity and hardness, strongly negative LSI. Remineralisation is not a finishing touch, it is part of the process, and the tool will not report a compliant design without it.
• The concentrate is the design constraint at an inland site. The nitrogen is not destroyed, it is concentrated. The kg N/day figure below is what has to go somewhere. If that somewhere is land or a soakage field in the same catchment, the nitrogen returns to the aquifer.

• Every option is modelled independently against the source water on the Source Quality tab, using the configuration set on each design tab.
• The nitrogen fate column is the one that is easy to skip and should not be. Ion exchange and reverse osmosis produce compliant water and a concentrated nitrogen waste stream. They do not destroy nitrogen. At an inland site with no marine outfall, the waste stream usually returns to land, to ground, and to the aquifer.
• Two things here look similar and are not the same. On the Ion Exchange and Reverse Osmosis tabs, partial treatment treats a fraction of this bore and bypasses the rest of the same bore. Blending below mixes in a different source that already has lower nitrate. Both are weighted averages, which is why they feel alike, but one needs a treatment plant and the other needs a second bore.
• Cost bands are indicative and relative. Use the cost estimation tool for a CAPEX/OPEX/NPV comparison.

This is the only export point in the tool. Select the sections to include, then print to PDF, export the results to CSV, or save the full input state as JSON for reload later.