Groundwater a mysterious resource
By Dr Helen Rutter, Aqualinc
In an 1861 US court case considering the rights to groundwater use (Frazier vs Brown), the judge ruled that “the existence, origin, movement and course of such waters, and the causes which govern and direct their movements, are so secret, occult and concealed, that an attempt to administer any set of legal rules in respect to them would be involved in hopeless uncertainty, and would be, therefore, practically impossible”. However, in the years since, we have developed ways of understanding this mysterious resource, such that, though uncertainties remain, we can predict cause and effect with much greater confidence.
Groundwater and surface water are two components of a single resource: groundwater is the link between the land surface and surface waters. Rivers and streams that are maintained in the absence of rainfall are due to groundwater discharging into the surface waters. Land use changes that affect recharge water quality will impact on groundwater, which will ultimately be discharged to the surface or at the coast. In order to predict how surface water quality and quantity will respond to changes in groundwater quality and quantity, we need to understand the likely flow paths. But it is not only surface waters that are of concern. We rely heavily on groundwater for drinking water, irrigation, and other uses, and we need to understand how land use changes or climate variability and trends might impact on groundwater itself.
So, what has changed since 1861, such that we believe we can predict how groundwater, and surface waters, will respond to changes in abstraction, land use, or climate? We now routinely collect a huge amount of data on groundwater levels, groundwater quality, surface water flows and quality, and various aquifer properties. We are able to carry out tracer tests, where we introduce a tracer into the groundwater environment and test to see if it appears at another location. Age dating techniques have advanced such that we can start to understand the age of water at a specific location. However, all of this information presents us with an intractable problem, with groundwater levels, river flows, and water quality and age, which all vary in space and time. In order to make sense of this multitude of data, and with the advent of modern computing, we can use complex 3-D computer models to model groundwater flow and contaminant transport, and predict the effects of changing inputs into the system.
Groundwater modelling provides us with a way of investigating management options, but underlying the model is the need for a thorough understanding of the system, based on multiple lines of evidence. A lack of understanding of how groundwater and surface water interact can result in a range of issues, such as microbial contamination of groundwater for drinking water supply, or to eutrophication of swimming holes. Groundwater management may require a multi-disciplinary approach, with the skills to carry this out distributed across organisations.
For more information about groundwater in
Dr Helen Rutter, Aqualinc’s South Island groundwater expert, recently presented her work at the February Speaker’s Science Forum, a well-attended series of research presentations for Members of Parliament organised by The Royal Society of New Zealand in partnership with IRANZ, Science New Zealand and Universities New Zealand. The February Forum was on Understanding New
For a copy of Helen’s presentation to The Speaker’s Science Forum, please download the PDF.