Special Topic: Hydraulics & Quality of Groundwater

Description

Groundwater is a massive but somewhat mysterious resource that may constitute the greater part of the planets fresh water resource. In NZ it accounts for about 30% of fresh water use, while Christchurch itself is supplied entirely by groundwater from its underlying system of artesian aquifers. Compared to surface water groundwater is a mysterious resource with uncertainties that are uncomfortably large. It is also a delicate resource, and once contaminated stays contaminated, containment being usually the only option.

The aim of this course is to give you a sound appreciation of the mechanisms of groundwater movement, and understanding of contaminant hydrogeology and the underlying chemistry. The breakdown of the course will be roughly 2/3:1/3 hydraulics/quality. The first part of the course will build on Basic Fluid Mechanics ENCN342, in that we will emphasise the underlying physical mechanisms and essential mathematics. This course will focus less on formulae for assessment, and more on the processes and uncertainties. The course will introduce novel aspects such as:

a. Uncertainty quantification. Usually our knowledge of aquifer dynamics comes from fitting
often too-simple models of 2-dimensional groundwater flow (e.g., like those based on heat
equations or Darcys Law) to aquifer test data. Uncertainty quantification is about quantifying
uncertainties in model fits, we will Monte Carlo Methods to quantify uncertainties, and
explore what this means for model predictions. The lessons here are generic: whenever we
want to interpret data using a mathematical model, quantifying the uncertainties (and hence
assessing the reliability of model predictions) is a fundamental task.

b. Numerical hydraulic modelling from the ground up. In engineering practice the numerical
modelling of groundwater phenomena is done using black box packages using either the finite
difference method or the finite element method. In this course we will take a ground-up
approach and use Matab to solve a simple groundwater flow model using the finite difference
method since it is the most straightforward approach (although we will use a formulation
which makes it easy to move onto the finite element method). This is a generic skill for
anyone interested in numerical modelling (e.g. structural models, seismic wave propagation,
contaminant transport).

c. Poroelasticity and seismic waves in aquifers. Poroelasticity refers to the complex interactions
between fluid (groundwater) and solid (aquifer matrix); as encountered in geotechnical
engineering and the effective stress/pore water pressure concept of Terzaghi, for example.
Usually fluid motion is separated from the solid matrix, whereas an aquifer constitutes a dynamic
coupled system. We will explore poroelastic phenomena and derive the poroelastic
wave equation, and look at simple solutions.

d. Contaminant transport and retardation processes. Hydraulic models can be coupled with
geochemical models to predict contaminant mobility in the groundwater system. Contaminant
transport (how far and how fast contaminant can move in a groundwater system) depends on
the chemical nature of the contaminant as well as the characteristics of the host rock and the
aquifer itself. We will explore the processes that bind and release contaminants in aquifers,
and how they are modelled.

e. The relationship between groundwater and spring flow. Where the water table intersects
the ground surface, we can either get springs recharging spring-fed streams and rivers, or
alternatively recharge of groundwaters from the surface water system. We will investigate the
relationship between Mid-Canterbury aquifers and the coastal spring fed streams, including
those of urban Christchurch.

f. Salinity intrusion into coastal groundwaters. Salinity intrusion into a freshwater aquifer
near the coast commonly causes loss of useful water yield from a well. We will explore the
nature of the subsurface seawater/freshwater interface, and how this affected by groundwater
extraction.