Cross-sections, Contouring and Conceptual Hydrogeology Models Assignment

Assignment Task

Introduction

During this prac you will learn the key skills required of practising hydrogeologists. The main tasks carried out by these professionals are to:

• Work with bore logs , well construction details and other geological data;
• Conduct measurement of water levels in monitoring wells and interpret these data to determine groundwater flow directions, flow rates and interaction with surface water;
• Conduct groundwater sampling and analysis for chemical components in water, interpreting these results to understand the controls on groundwater quality.

Together, these are the building blocks for a conceptual hydrogeological model. These conceptual models help answer questions about groundwater flow, groundwater contamination and anthropogenic effects on groundwater. A conceptual hydrogeological model can be used to estimate groundwater flow directions and travel times, determine hydraulic connections between different aquifer layers and surface water bodies, and determine impacts of groundwater pumping, mining and other activities. In some cases a conceptual model may then be converted into a numerical groundwater model (this is a topic covered in CIVE1122 Advanced Hydrogeology).

In this practical you will use data provided to develop a conceptual hydrogeological model to help understand a groundwater contamination problem. Parts of the data in this prac are taken from a real-life contamination investigation. The names, locations and other details have been changed for confidentiality reasons.

The task

You have been commissioned to investigate possible contamination of groundwater by coal seam gas operations, which commenced in this area in 2011. The investigation was initiated when a landowner noticed an unusual smell in water from his well, used for drinking water supply. Testing revealed elevated concentrations of boron and methane gas in the water. A new supply well was drilled near the old bore to a deeper level, to provide an alternative supply of water.

Key features on the site that may be of relevance to the potential contamination problem:

Two major coal seam gas production wells exist, and in both of these, hydraulic fracturing (‘fracking’) has been used to improve gas production rates. These wells are sealed down to the depth of the major coal seams, (approximately 600m below the surface) in an attempt to prevent connection and cross-contamination with shallow aquifer units.

The gas extraction and hydraulic fracturing result in production of large volumes of waste water, with variable quality. This waste water requires treatment and removal from the site. A water treatment plant is located in the north of the area, which processes the waste water produced during drilling, hydraulic fracturing and gas production. Water from this treatment plant (which was sampled, with results shown below) was inadvertently spilt into the river on at least one occasion. A water storage dam is also present near the main gas production well and treatment plant, this may have in the past been used to store excess water produced during gas well development, or excess water that has undergone treatment from the water treatment plant. It is not known if there is any lining at the base of the storage dam.

Tasks in this prac

1. Produce a cross section based on the bore logs and indicate (by labelling the cross section) which parts of the subsurface represent aquifers or aquitards. In your cross section, use appropriate vertical exaggeration and scale, space the bores correctly according to map distances (either absolute map distance or projected distance along a straight line), and provide a full legend

2. Produce contour map(s) of water levels for any relevant aquifer(s), selecting an appropriate contour interval. Indicate the main groundwater flow directions in map view, using clearly marked arrows. Remember the rules regarding the relationship between equipotential contours and flow directions

3. Calculate average horizontal hydraulic gradients between the following features:

• Water Treatment Plant to River
• Water storage dam and the water supply bore(s)
• Average gradient across the site (choose two points along one or more flow lines)

Using these gradients, estimate groundwater travel times between the two points in each case using the table provided. An effective porosity of 0.3 (30%) can be assumed for all of the geological materials; while estimates of hydraulic conductivity for various geological materials are provided (.pdf titled ‘conductivity prop’)

Also, calculate the vertical hydraulic gradient, indicating flow direction, for one locality (choose a site of interest for the problem). What would be the approximate vertical travel time for water between the two points at this locaility if there was a connected flow path?

Based on your understanding of the site, answer the following questions:

1. What is the likely source and pathway of contamination to the old supply bore? Justify your answer on the basis of your conceptual hydrogeological model, and water quality data. What is the approximate travel time for any contaminants dissolved in groundwater to follow this pathway, assuming they travel at the same speed as the water, and that groundwater flow is steady state? Does this seem to be realistic given the length of time the CSG site has been operating? 

2. If wastewater from the water treatment plant was spilled into the creek, is it likely that the creek could have acted as a pathway for contamination to the old supply bore? What about the new supply bore? Explain why or why not clearly 

3. What is the danger to the new supply well of contamination under current conditions? Under what circumstances or by what pathways may contamination of the new supply bore occur in the future?

4. Is the river in any danger of contamination from groundwater (if so, where?)

5. Is there evidence that hydraulic fracturing at Wells A or B could have caused contamination of groundwater in the region? 

This Engineering has been solved by our PhD Experts at My Uni Paper.