Gravel bed rivers research update, june 2022.

The information below was written after a presentation was made to the MDC Environment Committee on June 15th, 2022.

Gravel Bed Rivers (GBR) research on Wairau River and Aquifer.

A presentation was made to the MDC Environment Committee on June 15th providing them with the latest information arising from the GBR research program which started in 2019. The purpose of this latest report was "To provide an update to the Committee on research results from the national Gravel Bed Rivers project investigating the hydraulic connection between braided gravel rivers and alluvial aquifers."
In their Executive Summary they state - "The prime reasons for the ongoing decline in Wairau Aquifer well levels is less Wairau River water
available for recharge and a reduction in the capacity of the natural pathways to move water from the river into the aquifer. This is compounded by demand in some drier seasons."

This is not really an unexpected conclusion when we know the river has been modified severely from it's original natural course and is now contained within stopbanks. It is interesting to note that the research team think that water extraction from the river and aquifer is not a major contributor to the ongoing decline trend and that it is the reduced recharge that is of the greatest significance. 
The research team have proposed  - "Having established a conceptual model of how the river-groundwater system work, the river-groundwater system will be modelled more accurately than previously. A model will be used to test the sensitivity of the river-groundwater water balance to riverbed elevation, scouring, and floodway width. The results will be used as a basis for a cost-benefit analysis to see how changes to current river management would impact the local economy."
If effective solutions can not be found to stop the decline in the aquifer the consequences for those growing and processing grapes and others who rely on this water for their operations and livelihoods will, in time, be considerable. Add to this the prediction that we are likely to experience hotter and drier summers due to global warming and it is not hard to imagine serious impacts for Marlborough in the decades ahead.

I recommend reading the full Executive Summary (Item 5 on the agenda) if you wish to understand more clearly the dynamics of the reduced aquifer recharge process proposed by the research team. 

It is of further concern to read that "The decline in Wairau Aquifer levels is consistent with widespread deepening of wells over the past
35 years at least. Deepening wells improves individuals access to groundwater but will not prevent aquifer fed springs from drying up as they rely on shallow groundwater breaking the surface for their existence."

At the same meeting MDC Groundwater Scientist Peter Davidson also presented the annual Groundwater Quantity State of Environment (Item 6 on the agenda) report. One bit of information from the report (page 12) stood out for me. Peter believes that "based on an extrapolation of the current rate of flow recession, Spring Creek will recede to State Highway 1 by about the year 2100 and by association all of the springs including in Blenheim." I see this as concerning information. One thing our communications with MDC staff have highlighted is that there is not enough evidence of the actual volumes of water being drawn out of the aquifer by water users to ascertain yet how much this water use is impacting the declining trend in the aquifer, as actual water use has only been metered for the last 5 years or so.

The lower Wairau aquifer has 3 Freshwater Management Units (FMU's). Levels are set, that if reached, will trigger restrictions for water users in those areas. For anyone interested you can view the Graphs showing the cut-off levels for the Northern (Wratts Rd), Central (Mills and Ford Rd) and Urban (Murphy's Rd) springs FMU's. You can also access the graphs showing long term data from the monitoring wells on the council website here. I have analysed some data supplied by MDC and it is interesting to note that in the dry years of 2015, 2019 and 2020 the aquifer level in the Northern (Wratts Rd) monitoring bore was only 50 - 60mm above the restriction level. This is the bore closest to the Spring Creek headwaters and therefore the best indicator of likely impacts on the springs. In communication with council staff we have learned that there are restrictions on all Wairau Aquifer Sectors except for the Lower Wairau and what they call the Recharge Sector of the main aquifer, which is a large proportion of the total aquifer. The reason the Recharge Sector has no restrictions currently is that MDC weren’t confident at the time of writing the MEP (Marlborough Environment Plan) that they had sufficient understanding of whether reducing cumulative abstraction would result in any benefits on downstream groundwater fed spring flows. They also say the pMEP restrictions are currently 100 % reliable but due to the declining trend in Wairau Aquifer levels restrictions are likely to become permanent at some point in the future, which is why MDC is focusing on what they call “alternative approaches to managing seasonal and boundary effects.” We are not clear what that actually means so will need to do some more research to learn more. It seems that any actions arising from the GBR research, to try and reduce or stop the declining trend in the aquifer are likely to be very expensive and to take decades to prove their worth. Suffice it to say it seems clear that this issue will be ongoing and not easy to resolve. The pMEP restriction regime is currently subject to appeals which should be heard some time in 2023.

Maia Hart has also done a good article on the GBR report in Stuff where she says - "New research suggests historic work to narrow the Wairau River could be contributing to declining levels in the recharge aquifer – one of Marlborough’s main water sources. The Wairau aquifer is the main groundwater system underlying the Wairau Plain and a source of irrigation, drinking and stock water. Water seeping from the Wairau River into the aquifer is the main ways it is recharged. Its levels have dropped since 1973, at rates unable to be explained by irrigation."