Section 4   Groundwater Sampling

The monthly groundwater sampling was carried out by the DoW South Coast Region. The sampling program was designed to obtain data about chemical properties and seasonal variations of the nutrient concentrations. The sampling program is described below:

4.1 Groundwater Level

The depth to watertable (SWL) was measured every month, and the result is shown in Appendix B. Depth to watertable ranges from over 1 m to 4.7 m. Watertable fluctuate with season, and ranges from 0.5 m near the foreshore to over 2.5 m far away from the foreshore. However, watertable in Bores WIG09 to WIG13 show much smaller seasonal fluctuations. Groundwater hydraulic gradients along transects need to be estimated for calculating nutrient loads.

4.2 Sampling

Groundwater samples were collected on a monthly basis. Each bore was purged in order to provide a representative groundwater sample. The purging consisted of removal of three "casing volumes" of groundwater. In some cases, the slotted section of a monitoring bore was in lower permeability strata and the monitoring bore was pumped dry three times prior to the sampling. The sample containers used were 500ml high density polyethylene bottles. The sample bottle and lid were thoroughly rinsed three times with the groundwater before the sample was collected. The sample bottle was filled completely to ensure that no air bubbles were present. The sample bottle was immediately transferred to the cooler box.

Laboratory analysis included major ions, nitrogen (TN, NO2, NO3 and NH4) and phosphorus (TP, PO4), pH and TDS. Appendix B shows groundwater quality diagrams.

4.3 Field Measurements

Field measurements were carried out using colorimetric test kits and portable meters. Conditions and procedures of the measurement were observed in accordance with the instruction manuals. The meters were calibrated against standard buffer solutions before usage. All electrodes of the meters were decontaminated using deionised water after each measurement. The parameters measured by portable meters included electrical conductivity (EC, in μS/cm), redox potential (Eh, in mV), pH and temperature.

4.4 Brief Data Interpretation

Groundwater salinity varies with location and season. WIG02 shows a typical salinity pattern. The salinity level is about 700 μS/cm in September, and is about 1,700 μS/cm between December and May. However, many bores, especially those having fresh groundwater (EC < 1000 μS/cm) in the summer, have stable salinity.

Groundwater pH changes significantly with location and season. It ranges from 4.5 to 6.7 and show very different seasonal trends between the bores.

The background groundwater nitrogen (TN) level is about 5 mg/L, with exception of Bores WIG04 (12 mg/L), WIG06 (6 mg/L), and WIG12 (9 mg/L). Nitrogen levels are generally higher in the winter.

The background phosphorous (TP) level is below 1 mg/L, with exception of WIG04 (1.2 mg/L). TP levels are generally higher in the winter.

Laboratory chemical results are shown in Appendix C. TKN (Total Kjel Nitrogen) and NH4 (ammonium) are generally the major form of nitrogen. TKN and NH4 are normally stable in the reduced condition with low pH and low redox, and indicate the samples are close to the source of nitrogen.

WIG04 has the highest TN level which comprises predominately NO3 which is in the oxidated Nitrogen form.

It is suggested that WIG04 be monitored closely and a duplicate sample should be take in the next round of sampling.

Wilson Inlet Groundwater Nutrient Monitoring
URS Australia Pty Ltd

Draft Report

Prepared for Department of Water, 18 December 2007