Supporting your success

La Grange groundwater assessmentTECHNICAL SUMMARY

Department ofAgriculture and Food

La Grange groundwater assessment TECHNICAL SUMMARY

The project aimed to define industry and culturally appropriate land and water resource development options, to support the expansion of irrigated agriculture in the region.

La Grange groundwater assessment The Department of Agriculture and Food, Western Australia (DAFWA) has completed a comprehensive assessment of the Broome Sandstone aquifer. This aquifer is the main source of groundwater in the La Grange area. The aquifer is a flat-lying, unconfined aquifer of variably consolidated sandstone, covering more than 30 000 square kilometres between Roebuck Bay and Mandora Marsh. It has a saturated thickness of more than 200 metres (m) near the coast and 50 gigalitres of groundwater per year is currently allocated for use.The assessment area focussed on the coastal zone within 50 kilometres of the Great Northern Highway.

How we assessed groundwater resourcesFrom 2012– 2016, DAFWA’s hydrology team collected ground and aerial data to assess the aquifer, and determine its capacity to support further irrigation development in the La Grange area.

DAFWA undertook the following main tasks:• a bore census to locate, survey, monitor and

sample about 300 existing bores to establish baseline conditions of the La Grange groundwater system

• with partners at CSIRO, acquisition and interpretation of 6000km of Airborne Electromagnetic (AEM) data to define aquifer extent and thickness, saltwater interface and other aquifer properties

• groundwater drilling (3300m) to confirm AEM results, provide stratigraphic and water level data, and develop a long-term groundwater monitoring network

Perth

Broome

La GrangeProject Area

0

Kilometers

500

• regional assessment of groundwater-dependant wetlands of the Broome Sandstone aquifer

• geochemical sampling of 150 bores to determine irrigation suitability and assess recharge using groundwater age, stable isotopes and tracers

• construction and calibration of a water balance model to assess potential impacts of further irrigation development on current and future groundwater users, and cultural and environmental values of the area

• construction of a depth to watertable interactive online map to help stakeholders in pre-feasibility planning for irrigation development

• creation of maps of areas most suitable for future irrigation development.

Assessment resultsWater qualityAquifer water quality in the area assessed as most suitable for irrigation, ranged from 280 to 600 milligrams per litre (mg/L) of total dissolved salts. The highest groundwater salinity measured during the drilling program was 103 000mg/L — three times more salty than seawater — at the base of the aquifer near Mandora Marsh. Some other inland sites and some near the coastal salt water interface had salinity levels above that suitable for irrigation and were not recommended for development.Bores assessed in the project area are shown in Figure 1. Figure 2 shows the Northwest Coastal Highway (red line), inland extent of the saltwater interface (yellow line) and the blue-green colours represent areas of lowest groundwater salinity as per AEM survey results. The numbered yellow dots indicate the DAFWA monitoring bores drilled for the project.

The La Grange Agriculture Opportunities project was established in 2012 to investigate opportunities to develop irrigated agriculture in the La Grange area, south of Broome, Western Australia.

Aquifer propertiesThe Broome Sandstone aquifer covers most of the La Grange area. The aquifer and watertable dip gently from the east towards the coast, where the aquifer is at least 200m thick. The aquifer can be divided into three main zones:• an upper unit of mainly sand and consolidated

sandstone, typically about 30m thick• a middle unit of high permeability, mostly

unconsolidated sandstone (usually 120m thick,

but 211m thick at one site), referred to as the ‘production zone’, often containing sections of coarse sands and/or gravels. Aquifer testing shows that the hydraulic conductivity of this zone may exceed 50m/day.

• a lower unit of moderate-to-low permeability sand and silt with an average thickness of 15m overlies weathered Jarlemai Siltstone (basement rock).

Geoffroy Bay

Gourdon Bay

Roebuck Bay

Lagrange Bay

Admiral Bay

Desault Bay

Port Smith

Barn Hill

Blue Hills

Colorado Outcrop

Eidibee Hill

Gingerah Hill

Ironstone Hill

Karrobridill Hill

Parda Hill

Mount Phire

Radi Hills

Stony Hill

Tree Hill

Willara Hill

Woods Hills

Yarrana Heights

Mount Morris

0 25 50 75 100

Kilometres

Roebuck

FrazierDowns

Thangoo

Mandora

Nita Downs

Shamrock

Anna Plains

ShamrockGardens

Shelamar

Eighty

Mile

Bea

ch

Gre

at

Nort

her

n

Hig

hway

Legend

Groundwater allocation areas

La Grange Project Area

Station Boundary

Bore (WIN sites)

Oil well

Landscape feature, hill, etc.

Reserve

Vacant Crown Land

Northernallocation area

Southernallocation area

Figure 1. La Grange groundwater assessment area

0 50

Kilometres

Sandstone withminimal

groundwater

Sandstone withminimal

groundwaterAEM

HighConductivity

LowConductivity

Salt water extent

DAFWA Monitoring Bore

Highway

Project Area

Figure 2. AEM survey results shown at a depth of about 50m below ground level and the 24 monitoring sites in La Grange

Groundwater age and rechargeCarbon-14 derived age data shows older waters at the base of the aquifer and younger waters closer to the watertable (Figures 3 and 4). Modern recharge was verified with isotope and tracer data. Rainfall recharge was determined to infiltrate after major thunderstorms (e.g. 60–100mm storms) and be largely absent during the dry season and after smaller rainfall events. Using Carbon-14 and other techniques, median recharge across the La Grange area is about 15mm per year.

Groundwater-dependent wetlandsThe drilling results confirmed the location of the watertable which was then used to identify wetlands that may rely on the Broome Sandstone aquifer. It was found that along the coast, most wetlands are groundwater dependent. It was also found that most wetlands away from the coast are not reliant on the Broome Sandstone aquifer and are likely to be dependent on perched groundwater resulting from localised rainfall-recharge. A notable exception is Munro Springs and the surrounding areas which is dependent on the Broome Sandstone aquifer.

Jarlemai Siltstone

Sea Coast

Salt

water

extent

0m

-100m

100m

-200m

-300m

0m 20km 40km 60km

Water Level

Fresh Water Broome Sandstone

Bore 9 Shallow (5.5m)2400mg/L

Bore 9 Deep (48m)240mg/L

1900 years

Bore 10 (180m)310mg/L

4300 yearsBore 13 (69m)

Bore 14 (208m)470mg/L

4200 years

De

pth

Coast

AEM

JarlemaiSiltstone

Salt WaterBroome

Sandstone

0m 105km16km

Salt waterinterface

Broome sandstoneaquifer

Broome sandstone dry orwith minimal water

0m

-200m

164km

Coast

La Grange ProjectArea Boundary

-400m

200m

Figure 3. Cross-section showing the shape of the Broome Sandstone aquifer based on AEM, drilling and hydrochemistry data

Figure 4. Western section of Figure 3, showing monitoring bores, aquifer salinity and Carbon-14 ages of groundwater

La Grange groundwater assessment TECHNICAL SUMMARY

The figures above show the extent of the Broome Sandstone, the saltwater interface and underlying Jarlemai Siltstone, against the AEM data. Details of water quality near selected monitoring bores demonstrates the local variability in aquifer salinity and groundwater age. The saturated thickness of the Broome Sandstone decreases to the east, and maybe absent or thin in some areas.

Saltwater interface (red for high salinity)

Variability in rock type and salinity

Jarlemai SiltstoneWatertable

Drilled bore sites and their relative depth

Bore depth (m), salinity (mg/L) and groundwater age (years)

Bore 9

Ongoing work A water balance model has been built to assess the aquifer and forecast future changes under various irrigation and climate scenarios.

The model can be used to test the impact of irrigation (aquifer pumping) using existing irrigation designs and extraction volumes, or it can be modified to test the impact of climate (e.g. effect of dry periods), and new pumping options.

The aquifer and water level data was combined into a web-based forecasting tool which allows users to explore depth to groundwater and aquifer thickness within La Grange.

Visit: www.agric.wa.gov.au/assessmentagricultural-expansion/la-grange-westernaustralia- groundwater-map

La Grange Western Australia groundwater map

The groundwater assessment has provided a thorough understanding of the Broome Sandstone aquifer so informed decisions can be made about future agricultural development. DAFWA worked closely with local pastoralists, horticultural businesses, Yawuru, Nyangumarta and Karajarri traditional owners, CSIRO, Department of Water, Groundwater Consulting Services, Innovative Groundwater Solutions and Kimberley Water.The La Grange groundwater assessment is part of the Regional Economic Development Water Opportunities project. This project is made possible by Royalties for Regions.

Important disclaimerThe Chief Executive Officer of the Department of Agriculture and Food and the State of Western Australia accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.Copyright © Western Australian Agriculture Authority, 2016

Department ofAgriculture and Food

More information

Dr Richard George Principal Research ScientistP: +61 (0)8 9780 6296E: richard.george@agric.wa.gov.au

Project legacy