Auckland Aquifers! What's the big deal?

It's hard to find good graphics or geological cross sections of Auckland. So here's one from somewhere else. The idea's the same though. It's all about groundwater, and underground water systems. When it rains, a lot of water goes into the ground. It percolates through rock and soil till it reaches the water table - the natural level of water you find when you dig down. This area is sometimes called an "unconfined aquifer" because that water flows sideways and downward. These are underground water flows. They are slow. Auckland's volcanic rock is fractured and cracked and contains lots of caves and spaces which are full of water. Many of these water volumes are "confined". Full of quite pure water. It's pure because it has flowed through rock to get there. Sort of like a natural filtration system. The volcanic rock beneath that is called bedrock.

This picture is from Hastings District Council website. Almost all of Hastings City's water comes from underground water sources. From aquifers. The graphic shows the water cycle. Evaporation, rain in the hills, some overland flow in the Heretaunga Plains, and then the water seeps underground, and percolates through and around various rock layers, till it gets into quite deep aquifers. Hastings Council pumps the city's water up from these aquifers, and then reticulates it to users.

Imagine this graphic is of Auckland. City in the background, soil on the surface (imagine those cacti are Pohutakawa Trees...). The water table is marked. There's a graphic showing rock and soil near the surface. Then beneath that is a saturated area full of groundwater. Below that is what is called the "upper aquifer", and so on...

Now let's imagine we dig a quarry into this. I show it here as "Quarry 1". It's reasonably shallow to start with. Stays above the water table mostly, so it is easy to dig the rock out (aggregate) without being troubled by water. That's why it's got a flat bottom....

But then the quarry operator wants more aggregate, and gets permission to pump out any groundwater that flows into the quarry. This means the quarry operator can dig down much deeper into the saturated groundwater system. He pumps out water, and digs out more rock. In doing so, of course the floor of the quarry gets nearer and nearer to the top of the upper aquifer. It means that there is much less rock between the bottom of the Quarry (Quarry 2) and the top of the aquifer.

So now the Quarry operator finds there's no more rock available to dig out. He's gone as far as he can go. The quarry can't expand outwards because by now there are houses all around it. So now he seeks permission to fill the quarry with cleanfill. People pay to dump their cleanfill in his empty quarry hole. You can see a purple pile of fill in this graphic at the bottom of the old quarry. There's no need now for the operator to pump groundwater from the quarry hole, and so groundwater seeps in from the walls of the quarry and rain comes down from above.

This water percolates through the cleanfill, leaching out anything that is mobile or soluble. This water has only a short distance to travel now, from the bottom of the quarry, before it gets into the top of the aquifer. Once it's in the aquifer it's free to flow wherever the aquifer water movements take it. The problem comes when there are contaminants dumped with the "cleanfill". Best practice suggests that cleanfill should be just that - the quarry operator should really be replacing like with like. Take out inert rock - put back inert rock. Then there will be confidence there is no risk to the aquifers. No health risks to those who draw water from the aquifer for drinking, or for emergency supplies. That's the big deal.