Supervising Scientist Division

Supervising Scientist Annual Report 2003 - 2004: Groundwater pathways

Supervising Scientist, Darwin, 2004
ISBN 0 642 24391 3
ISSN 0 158-4030

3 Environmental research and monitoring (continued)

3.5 Groundwater pathways

This programme was introduced during the year and is in the process of defining the research area. The groundwater theme will monitor and investigate contaminant movement in groundwater in the vicinity of uranium mining activities in the Alligator Rivers Region. Key activities in 2003-04 included:

It has been several years since the Nabarlek mine began the rehabilitation process and with the approaching rehabilitation of Ranger uranium mine, groundwater issues are gaining more prominence.

The long-term groundwater projects that are in place now are being revised and extended and 2004-05 will see the commencement of the expansion and further development of the groundwater theme.

3.6 Surface water monitoring

This programme monitors water quality in Alligator Rivers Region creeks in order to assess effects of mining upon ecosystem and human health. An integral part of this programme is the ongoing review and refinement of current water monitoring techniques and the development of new techniques.

Key activities in 2003-04 included:

3.6.1 Developing a simple tool for the continuous monitoring of trace metals in freshwater systems

The technique of diffusive gradients in thin films (DGT) is a unique method allowing for in situ measurement of the fraction of dissolved metals in waters that is available for uptake by aquatic organisms. It is this bioavailable fraction that is capable of causing a toxic response. In conventional water sampling methods, for example those used in the routine SSD environmental monitoring programme, the dissolved metal fraction is estimated by passing the water sample through a membrane filter with a pore size of 0.45 µm. However, this conventional sampling method has a number of limitations that the DGT technique potentially alleviates, including:

The potential of DGTs for continuous monitoring of trace metals in streams near the Ranger mine was investigated in collaboration with Charles Darwin University over the 2003-04 wet season.

A DGT sampler consists of a gel assembly that is enclosed in a piston-like plastic casing. The gel assembly contains a cation exchange resin that irreversibly binds a suite of heavy metals and a diffusive gel layer that controls the rate of mass transport into the DGT device. The gel layers are separated from the water sample by a membrane filter (0.45 µm) to exclude particulate matter (Figure 3.7). The DGT holder is suspended in the water being sampled.

Figure 3.7 Schematic diagram of a DGT sampler and arrangement of triplicates in the custom-designed holder deployed at each sampling site

Figure 3.7 Schematic diagram of a DGT sampler and arrangement of triplicates in the custom-designed holder deployed at each sampling site

Throughout deployment, metals continuously enter the DGT device and are bound and immobilised within the resin gel. The mass of metals bound to the resin gel is then chemically removed and measured using inductively coupled mass spectrometry. Using specific algorithms, the metal concentration in the water to which the DGT sampler was exposed can be calculated from the DGT-bound mass and expressed as a time-integrated metal concentration (averaged over the entire deployment period).

Field studies were performed to investigate the bioavailable aluminium, cadmium, copper, iron, manganese, uranium and zinc concentrations in Magela and Gulungul Creeks. Triplicate electrical conductivity devices were deployed for one-week periods at the same sites as used in the Supervising Scientist's water chemistry monitoring program. For the selected study sites, the time-integrated metal concentrations measured by electrical conductivity were compared with the average dissolved (<0.45 µm) concentrations measured in filtered water samples taken at the same times as deployment and removal of each batch of electrical conductivity devices.

Laboratory studies indicated that electrical conductivity (EC) significantly affects the reliability of the DGT method, with EC below a threshold of 250 µS/cm causing overestimation in the DGT measurement. Thus, the low EC of the creeks examined (typically <20 µS/cm) may have caused enhancement in DGT measurement, resulting in non-quantitative overestimation in the DGT measured bioavailable concentrations. The DGT-measured bioavailable concentrations for each metal studied were compared with current site specific trigger values for Magela Creek and default guidelines recommended in the national Water Quality Guidelines (WQG) (ANZECC and ARMCANZ 2000) (Table 3.1).

Table 3.1 Comparison of DGT measured bioavailable concentrations¹ (µg/L) with site-specific trigger values and national water quality guideline (WQG) defaults
    Cd Cu Zn U Al Mn
Trigger values²   0.06 1.0 2.4 5.8 27 32³
Average dissolved concentrations measured in filtered (0.45 µm) water samples Magela Creek upstream <D.L4 0.14 0.17 0.018 56 3.5
Magela Creek downstream <D.L 0.15 0.18 0.047 57 3.5
Gulungul Creek downstream <D.L 0.11 0.16 0.095 59 2.4
Average DGT measured bioavailable concentrations¹ Magela Creek upstream 0.0010 0.0080 0.28 0.0014 4.2 6.6
Magela Creek downstream 0.0010 0.0080 0.24 0.0043 3.1 6.4
Gulungul Creek downstream 0.0010 0.013 0.16 0.0081 3.1 2.3
1 DGT measured values were overestimates of the bioavailable concentrations due to the low EC of the creek water.
2 Trigger values are site specific for uranium (based on toxicity testing of local species) and manganese (based on reference site data). WQG freshwater default values for the protection of 99% of species are shown for other parameters.
3 Mn trigger value applies to Magela Creek only
4 D.L. refers to Detection Limit.

Table 3.1 shows that for all metals the DGT measured concentrations were well below the recommended trigger values. Thus, whilst DGT measurement in low EC solutions is enhanced, the overestimated concentrations measured were still below guideline values suggesting that the actual bioavailable concentrations of metals present in Magela and Gulungul Creeks (both upstream and downstream of Ranger) are low.

More developmental work is required to assess the full potential of the DGT method as a complementary water chemistry sampling technique. Should further developmental work prove successful, this method would meet the requirements for long-term monitoring, as it is a passive, in situ, low maintenance technique. The method may also be suitable for assessing the success of minesite rehabilitation and could potentially replace grab sampling as a method of monitoring metals in surface waters.