Monday, 7 April 2014

Where Does the Groundwater Flow?

There has been renewed interest in groundwater resources in the Northern Rivers of late. In part this is due to peoples concern about "unconventional" gas exploration and production in the area. Surprisingly, less known is the release of Rous Water's Future Water Strategy which includes groundwater as first on the list for new water sources. Rous Water is a major bulk drinking water supplier in the region. I've previously covered an area within the coastal sands groundwater source called the Woodburn Sands but this was a cursory look and I'd not covered where the groundwater actually goes.

Groundwaters do not exist as an underground lake in our region
Image courtesy of International Association of Hydrologists
Groundwater is often seen as a bit of an unknown, a black box, or some kind of underground lake (see the cartoon). It is quite difficult to observe and therefore people can get the wrong idea of what goes on underground.

One area that is not understood is that groundwater usually discharges somewhere. Sometimes groundwater discharge is obvious through springs. But where it intersects with permanent surface water it is much less obvious. The Evans Head area is a good example of where discharge from the Woodburn Sands aquifer and broader Coastal Sands aquifers is concealed.
Spring-fed creek on Chinaman's Beach.

While walking along Chinaman's Beach south of Evans Head during a recent long dry spell, I couldn't help notice the dark coloured water flowing over parts of the beach. This is one of those discharge areas I'm talking about (most people might be more used to seeing freshwater flowing over a beach from contaminated urban stormwater drains). The coastal sands above Chinaman's Beach holds groundwater and slowly discharges it at the beach. The dark colour of the water is from dissolved humic matter from coastal vegetation soaking into the sand. Tasting the water it was apparent there was no salt in it and understanding the groundwater area I knew it was clean. The springs I observed on Chinaman's Beach were obvious areas of groundwater discharge. The vegetation in the springs was lush and clearly reliant on the groundwater. This is formally known as as groundwater dependent ecosystems.

The lesser known discharge is not all through visible springs like those on Chinaman's Beach. Much of the discharge from the coastal sands aquifers is actually concealed by the sea. It might be a surprise to many in some areas just off the coast there are zones with freshwater. The amount of water that can be discharged underground into the sea can exceed the discharge from terrestrial springs (e.g. Santos et al. 2009). These are the undersea equivalent of the Chinaman's beach springs. This is interesting from a aquatic ecology point of view because it may mean that there are ecosystems in the ocean that are dependent on freshwater! That is, groundwater dependent ecosystems in the sea.

Groundwater is an interesting feature of our region. It is a source of drinking water, irrigation water and even industrial water. It is often important as some ecosystems are dependent on it. It is also surprising since ecosystems can be dependent on fresh groundwater even when out to sea.

Postscript: about a month after this blog post a story emerged in the local newspaper about sinkholes or zones of quicksand on Chinamans Beach. These quicksand 'pits' look just like typical groundwater discharge areas. The Northern Star article can be found here.

*Santos, I.R, Burnett, W.C., Chanton, J., Dimova, N. & Patterson, R. (2009). Land or Ocean?: Assessing the driving forces of submarine  groundwater discharge at a coastal site in the Gulf of Mexico. Journal of Geophysical Research. vol114.


  1. Interesting little post Rod,
    the reference is quite amusing too, as most people from the Northern Rivers will think you have refered to a gas company rather than a Mexican scientist. lol

    1. Hi Dylan,

      I know it is funny. Santos is actually a SCU academic at the moment. He has told me exactly the same irony.

      Although the research was in the gulf of Mexico he is actually from Brazil. Even the northern rivers seems to be connected with the rest of the world these days.

  2. Very interesting post, Rod. There is a place in Wangi where ground water comes to the surface beside the lake here. It was the first fresh water supply for settlers here - and for the Aborigines long before that.

    1. That sounds like a very good example of how important groundwater discharge areas are. I wonder if there are many groundwater bores in your area that tap that aquifer.

  3. Fascinating issue, fascinating processes. Have there been any plans to tap the Woodburn Sands? What about groundwater resources in places like the Alstonville Plateau? Are there any detailed assessments of that area, water quality, inputs and extraction?

    Over the border there is a bit of conflict on Tamborine Mountain and other plateaus with local residents vs others who are pumping water for sale off mountain; private bores used on property vs simple water extraction and removal from the mountain. Lots of bores sunk there over many decades, must be hard to keep a handle on such use. Apparently, Tamborine is not really well assessed for sustainable yield, and is not formally gazetted as an aquifer by the relevant department. Love to read your opinion on this.

    1. Hi Nick,

      There are already some groundwater supply bores in the Woodburn Sands and Alstonville Plateau (as well as numerous private bores). I understand that the new Coastal Sands Water Sharing Plan to be released by the state government in a few months has a sizable allocation for drinking water supply too. The Woodburn Sands are generally regarded as having more capacity to develop without affecting discharge (too much).

      The Alstonville Plateau groundwater sources are over allocated and during droughts they are severely drawn down.

      I'm afraid that I'm not in a position to say much more about the allocation of groundwater sources of the area. However, there will me more said in the near future assuming the outcome of the future water strategy and other investigations that are ongoing.

  4. Hi Rod,

    I remember a time at Suffolk Park when there were severe water restrictions and anyone who didn't have access to bore water basically had a dead lawn and those who did had to put a sign up to save council having to check whether water was being used illegally. Many questions were raised by concerned/jealous neighbours about its use. One person claimed that for every metre the fresh water table was lowered, salt water would move inland up to six metres.

    Is there any merit to this?

    On another note; being a surfer, it is common to see fresh water coming through the sand, particularly at low tide. This fresh meeting salt is often where pipis proliferate.

    1. Hi Dylan,

      Although the issue of sea water getting into groundwater is certainly legitimate, be careful not to visualize groundwater as a lake. Bores will have a cone of draw-down this means that the water level in bore part of the aquifer though connected to another is actually lower than the rest of the aquifer.

      An issue with salt wedge development arises when multiple bores are over extracted and recharge is limited. This usually takes some time. Because the transmissivity in a lot of the coastal sands is within certain limits and because the aquifer volume is usually high the problems with salt wedges is probably limited.

      It is also more of a problem closer to the coast. For example draw down in a coastal zone may cause salt wedge issues whereas the same draw down further inland is unlikely to cause measurable salt water transmission.

      I think this issue probably requires a few future blog posts.

    2. Hi Rod,

      Yes, I was only refering to near coastal environments and thanks Rod for your informative answer. Underground water really does work in mysterious ways.

      I seem to remember concerns regarding the Botany Bay industrial centre and down-gradient groundwater supplies and they found that the water was actually not contaminated due natural filtration.

      More blogs on groundwater? yes please!

    3. More blogs on groundwater coming up.