Fisherman Islands Groundwater Investigation. Stage 2 - Hydrogeological Assessment

January 2000

Prepared by: Coffey Geosciences

The Port of Brisbane Corporation (PBC) recently established a Des Norske Veritas Level 2 Environmental Management System (EMS) for the Fisherman Islands port area. During development of the EMS, PBC identified limitations with regard to the groundwater database. Coffey Geosciences Pty Ltd (Coffey) were commissioned by PBC to install further groundwater monitoring bores and assess the hydrogeological regime of Fisherman Islands. This report presents results of the second stage of investigations including a detailed hydrogeological assessment of Fisherman Islands, an assessment of baseline groundwater quality and identification of possible contaminated areas, and recommendations for the implementation and management of a long term groundwater monitoring programme.

Fisherman Islands Layout, Monitoring Bore and Surface Water Sampling Locations

The hydrogeological regime of Fisherman Islands is complex. The area is located at the mouth of the Brisbane River, and lies within the Petrie Basin among tidal flats dominated by Quaternary sands, silts and clays. The islands have been progressively developed through land reclamation processes using dredged material deposited in a seawater slurry within segregated paddocks. These reclaimed sediments represent the major aquifer sequence of Fisherman Islands and are characterised by:

fine to coarse grained, unconsolidated saturated marine sands with interbedded silts and common to abundant shell fragments;
the thickness of the aquifer sequence is generally 3m to 4m across the islands;
sands generally outcrop at surface, although silty topsoils are not uncommon;
the base of the sands is identified by either massive organic marine clays, or by silty basal sections of reclamation paddocks; and,
hydraulic conductivity of the reclaimed sediments is very high.

Recharge to the aquifer is dependant on local site development. Areas of extensive hard stand and well developed drainage systems (such as the port facility) inhibit recharge of the groundwater system and promote runoff to local drainage systems, whereas undeveloped areas with sands (the aquifer) exposed at surface provide good conditions for groundwater recharge following rainfall.

Groundwater levels across Fisherman Islands range from 0m at the coast to around 6mPD (Port Datum) towards the centre of the island. Groundwater levels indicate two flow directions, with primary flow away from the central mound towards the coast, and an internal flow component towards an unlined drain adjacent to the rail line and discharging to the Brisbane River. Contoured groundwater level data, the spatial variation of salinity, and general trends in groundwater quality show the development of a distinct freshwater mound across Fisherman Islands. Tidal influence on groundwater levels is minimal, with a maximum variation of 10mm per tide cycle.

Groundwater discharge to surface drainage and surface water storages is occurring across Fisherman Islands. Groundwater provides baseflow to the unlined drain adjacent to the rail line, and a component of the baseflow to the stormwater drain near the Smith Brothers container storage terminal. Surface water storages are generally windows or depressions in the groundwater table.

Groundwater discharge to many surface water drains and impoundments is occurring across Fisherman Islands. Groundwater appears to provide the majority of baseflow to the unlined drain adjacent to the rail line, and a component of baseflow to the stormwater drain adjacent to the Smith Brothers container storage terminal. Groundwater / surface water interactions across Fisherman Islands can be summarised as follows:

Lined and unlined stormwater drains: groundwater discharge to stormwater drainage is occurring across Fisherman Islands;
The Rail Loop Pond: groundwater / surface water interactions associated with the Rail Loop Pond are complex. The system receives limited groundwater inflow from all sides. The only outflow is from apparent seepage near the unlined drain adjacent to the rail line. Evaporation appears to be the main source of water loss from this pond. Surface water flushing, seepage to the unlined drain and groundwater discharge are not of a sufficient magnitude to satisfactorily flush the system;
The Environmental Lake: the Environmental Lake receives both groundwater through flow and sporadic flushing by surface runoff and from the stormwater drain adjacent to the rail line during periods of high rainfall.
The Ornamental Lake: There is insufficient catchment for runoff collection, nor sufficient groundwater through flow to adequately flush this system.

Results of the analysis of groundwater samples indicate that:

groundwater is slightly alkaline, with pH ranging from 7.5 to 8.3;
EC is highly variable across Fisherman Islands, and is influenced by seawater or evaporative concentration within old reclamation paddocks;
groundwater can be categorised (grouped) as NaCl dominated, CO3/HCO3 dominated, or a transitional type between the two.

Groundwater quality appears to be influenced by reclamation processes, location with respect to the coast, and residence time (to enable in-situ dilution). An analysis of 60 elements was undertaken on a subset of eight groundwater samples. Results show concentration of various elements in areas near recent and current reclamation work. Additional chemical analysis for TPH/BTEX at four locations showed no anomalous results.

The nature of the alkaline environment is unclear, although it may be attributed to:

the generation of ammonia as a by-product of bacteriological breakdown of organic matter; or,
oxidation of shell fragments at surface to form calcium hydroxide.

Surface water quality is not strongly correlated with groundwater quality. Surface waters are alkaline, have elevated EC and TDS values, and are NaCl dominated. Variation in hydrochemistry between groundwater and surface water may be attributed to several factors, including:

susceptibility of enclosed surface water bodies to evaporative concentration of elements;
groundwater in constant contact with sand and clay particles (susceptible to chemical reactions such as sorption);
wind borne sea spray may influence TDS and EC levels of surface water directly; and,
the level of sealing within the beds of surface water impoundments (silt or clay) may inhibit groundwater through flow and hence possible dilution of surface water with “fresher” groundwater.

Observations made during site inspections indicate that operations on Fisherman Islands are well controlled with mitigation measures in place to manage potential environmental impacts. In the long term, the mitigation measures in place to protect groundwater quality should be maintained and the standards of environmental protection set by PBC should continue to be applied to any new development.

Coffey provides the following recommendations for the Fisherman Islands groundwater monitoring programme:

groundwater levels in all monitoring bores should be monitored on a weekly or fortnightly basis;
groundwater and surface water samples should be collected and analysed on a quarterly basis;
additional surface water sampling locations should be incorporated into the regional monitoring programme; and,
any further groundwater monitoring facilities installed as Fisherman Islands reclamation works progress should be incorporated into the regional monitoring programme.

Recommended additional surface water sampling locations are:

two sites along the unlined drain adjacent to the rail line near MW3 and MW12;
one site from the lined stormwater drain adjacent to the Smith Brothers container facility; and,
one site from the unlined drain connecting the stormwater drain adjacent to the rail line with the Brisbane River.

Additional monitoring bores are recommended as follows:

along the unlined drain adjacent to the rail line half way between MW12 and the entry point into the Environmental Pond;
between the Rail Loop Pond and the unlined stormwater drain adjacent to the rail line to assess seepage between the two; and,
at the north-eastern corner of Fisherman Islands following completion of currently active reclamation work.

Sampling to date (Dames & Moore 1999, and Coffey 1999) does not provide a detailed “snapshot” of water quality across Fisherman Islands in terms of consistent analytical parameters in a comparable time period. It is therefore recommended that a round of sampling be undertaken with all monitoring bores and surface waters analysed for the full suite of basic water quality indicators and toxicants to establish comprehensive baseline conditions. Subsequent groundwater sampling and analysis could then be scaled back to include appropriate sampling sites and analytical suites, the applicability of which can be reviewed each quarter.

The recommended analyte suite for groundwater and surface water sampling rounds are as follows:

Basic Water Quality Indicators: pH, EC, TDS, major cations and anions, Cl, NH4, N, B & F;
Toxicants: Sb, As, Ba; B, Cd, Cr, Co, Cu, CN, Fe, Pb, Hg, Ni, P, Se, Ag, Sr, Sn & Zn; and
Hydrocarbons: TPH & BTEX.

Analytical laboratory reports of hydrochemical analyses should be reviewed each quarter to enable early detection and assessment of anomalous results, and identification of potential contamination. Detailed assessment and reporting of groundwater monitoring should be undertaken on an annual basis and would include an evaluation of groundwater levels and recharge mechanisms.

The white “scum” present in the unlined drain adjacent to the Smith Brothers container terminal should be sampled and identified to assess possible sources and, if required, enable implementation of procedures to manage or dispose of the material appropriately. Depending on the water quality in the lined drain, it may be advisable to install a monitoring bore at this location.

In regard to management of surface water storages, the following summarises the recommended course of action:

stormwater drainages (lined or unlined) are susceptible to groundwater inflow across the site. While lining drains will reduce the magnitude of inflows, there is generally a driving head (groundwater levels greater than drain water levels) across the site, and thus complete prevention of groundwater inflow is not possible;
it is recommended that the rate and quality of seepage from the Rail Loop Pond to the unlined drain adjacent to the rail line be assessed. Furthermore, the nature of the pond base in regard to sealing should be evaluated; and,
the ornamental pond is insufficiently flushed by surface runoff or groundwater inflow resulting in a general degradation in water quality that is unlikely to improve without considerable physical alterations. Consideration should be given to backfilling this pond.

A more thorough assessment of groundwater impact on stormwater discharge quality is recommended. The installation of automated monitoring and sampling equipment in stormwater drains would allow previously outlined ‘static’ monitoring of water quality in these systems to be complemented with ‘dynamic’ monitoring associated with major runoff events.

Recommended locations include;

the stormwater drain adjacent to the rail line near the entry point to the Environmental Lake;
from the connecting drain between the rail line and the Brisbane River, ideally located where the open drain becomes an enclosed system; and,
from the unlined drain near the Smith Brothers container facility, located above any tidal influence.

Salinity profiling is recommended to define the fresh water / salt water interactions. Recommended locations include MW1, MW 2, MW 3, MW4, MW5, MW8, MW9, MW14, MW15, MW16, MW17 and MW18.

Mitigation measures currently in place to protect groundwater quality should be maintained and standards of environmental protection set by PBC should continue to be applied to any new sites developed.

As discharge of groundwater to surface water drainages and storages is occurring across Fisherman Islands, regular monitoring of groundwater quality needs to be undertaken for early identification of environmental issues across the site.