Chapter 5 Groundwater Quality and Hydrodynamics of an Acid Sulfate Soil Backswamp 5.1 Introduction Intensive drainage of coastal floodplains has been the main factor contributing to the oxidation of acid sulfate soils (ASS) (Indraratna, Blunden & Nethery 1999; Sammut, White & Melville 1996). Drainage of backswamps removes surface water, thereby increasing the effect of evapotranspiration on watertable drawdown (Blunden & Indraratna 2000) and transportation of acidic salts to the soil surface through capillary action (Lin, Melville & Hafer 1995; Rosicky et al. 2006). During high rainfall events the oxidation products (i.e. metals and acid) are mobilised into the groundwater, and acid salts at the sediment surface are dissolved directly into the surface water (Green et al. 2006). The high efficiency of drains readily transports the acidic groundwater and surface water into the estuary and can have a detrimental impact on downstream habitats (Cook et al. 2000). The quality of water discharged from a degraded ASS wetland is controlled by the water balance, which over a given time period is: P + I + Li = E + Lo + D + S (Equation 5.1) where P is precipitation, I is irrigation, Li is the lateral inflow of water, E is evapotranspiration, Lo is surface and subsurface lateral outflow of water, D is the drainage to the watertable and S is the change in soil moisture storage above the watertable (Indraratna, Tularam & Blunden 2001; Sammut, White & Melville 1996; White et al. 1997). While this equation may give a general overview of sulfidic floodplain hydrology, an understanding of site-specific details is required for effective remediation and management of degraded ASS wetlands. The impact of drainage-induced oxidation and the transfer of products is dependent upon the local geomorphology, the drainage network, watertable dynamics and antecedent conditions (Kinsela & Melville 2004). Furthermore, to effectively manage ASS it is essential to know: i) the depth of the sulfidic layer; ii) watertable dynamics relative to the sulfidic layer and; iii) the effect of drain management and climate on the water balance, watertable dynamics and the export of acid and heavy metals (White et al. 1997). 106 Chapter 5: Groundwater Quality and Hydrodynamics Re-establishing tidal exchange and/or ponding water on the floodplain are common techniques used to rehabilitate degraded floodplain wetlands. Research by Indratana, Tularam and Blunden (2001) on the Shoalhaven floodplain on the south coast of New South Wales (NSW) showed that ponding water via in-drain weirs was effective in the management of ASS. Ponding water within the drain raises the watertable to submerge exposed ASS and prevents further oxidation and acid production. Where the backswamp can also be reflooded and surface water cover maintained, evapotranspiration and drawing of acid salts to the soil surface is also reduced. Restoring tidal exchange may provide an added benefit to the rehabilitation process by neutralising acidic products, as carbonates and bicarbonates naturally present in the estuarine water can act as a buffer (Indraratna, Glamore & Tularam 2002). However, the effectiveness of this process is dependent upon a number of factors including flushing dynamics, groundwater transport and acid production rates (Glamore & Indraratna 2004). The aim of this chapter was to examine the groundwater characteristics of Little Broadwater following rehabilitation. Specific objectives of the study were to: i) investigate spatial and temporal variability of groundwater chemistry; ii) quantify the impact of tidal exchange management and climate on groundwater quality; and iii) determine if there were linkages between groundwater and surface water. 5.2 Materials and methodology 5.2.1 Acid sulfate soils at Little Broadwater The soils at Little Broadwater are typical ASS, characterised by strong acidity, poor drainage, salinity and sodicity (Morand 2001). The average depth of sulfidic sediment at Little Broadwater was 0.39 m, with potential ASS (PASS) below -0.90 m AHD, actual ASS (AASS) present between -0.90 m AHD and -0.40 m AHD, and a newer layer of PASS at -0.19 m AHD to -0.14 m AHD (Wilkinson 2003). It was estimated by Morand (2002) that the -1 soil profile had an existing acidity of 20.77 kg tonne H2SO4. Scalds were present in the north next to Mantons Road and in the south prior to the start, and during the first 6 to 8 months, of the water quality study (Figure 5.1). Iron and salt crystals were present on the soil surface at both scalds prior to the start of the longer-term study in April 2005 with no vegetation cover present. The scalds were submerged from December 2005 onwards. 107 Chapter 5: Groundwater Quality and Hydrodynamics Figure 5.1: Example of scalding at Little Broadwater prior to the start of the study. 5.2.2 Field equipment and monitoring Prior to re-establishing tidal exchange and reflooding the wetland, groundwater was monitored by Clarence River County Council (CRCC, now Clarence Valley Council (CVC)) from mid-June to mid-November 2002, to monitor short-term watertable dynamics and collect data on groundwater quality. A piezometer with a water level logger was located in the southern region of the wetland and another two groundwater sites were located in the northern region (Figure 5.2). All three of these sites are now permanently inundated. Water samples were collected from the southern piezometer in October 2002 and from the northern groundwater sites in November 2002. All samples were analysed for pH, electrical - 2- conductivity (EC), chloride (Cl ), sulfate (SO4 ), soluble iron (Fe) and soluble aluminium (Al) at the Environmental Analysis Laboratory, Southern Cross University, using standard methods according to APHA (1998). Groundwater monitoring post-restoration of tidal exchange was designed to investigate the seasonal and spatial variation of groundwater quality, examine watertable dynamics, and assess interactions between surface water and groundwater. Monitoring of the groundwater sites commenced in April 2005 and continued to February 2007 (referenced as Apr05, Jun 05, etc.), and was conducted in conjunction with surface water monitoring (see Chapter 4). Six piezometer wells were installed around the wetland, with P1 in the north located at the toe of a hill approximately 90 m from the wetland edge, and the remaining five (P2 to P6) located on the edge of the wetland (Figure 5.3). The wetland edge was defined as the high water mark in non-flood periods, which was determined as the boundary between pasture and bare ground 108 Chapter 5: Groundwater Quality and Hydrodynamics from previous inundation (Figure 5.4). The holes were augered to a depth of 3 m (P1-P4) or 5 m (P5 and P6), using a Bunyip auger system fitted with 75 mm auger flights. The depth of the wells was determined by the length of the water level loggers available for the study. The piezometers wells were 50 nb Class 9 pressure PVC pipe at lengths of 4 m (P1-P4) or 6 m (P5 and P6). The bottom metre was drilled on 4 sides with 5 mm holes at 50 mm spacing, and a PVC cap was glued to the bottom of the pipe (Figure 5.5). The pipe was then placed in the well and the hole was backfilled with 7 mm blue metal aggregate up to 600 mm from the surface, and then powdered bentonite mixed with some of the soil from the well was used to fill to the surface. A cement cap was placed around the pipe at the ground surface, and was domed on the top to allow water to runoff the top of the well. One metre of pipe was left above the ground to attach a cattle frame (Figure 5.5). The piezometers were capped when not in use, and surveyed to m AHD at a later date. Figure 5.2: Location of Clarence Valley Council (CVC) groundwater sites prior to restoring tidal exchange. 109 Chapter 5: Groundwater Quality and Hydrodynamics Figure 5.3: Location of groundwater and surface water sites at Little Broadwater. Figure 5.4: Example of location of wetland edge piezometers at the boundary between pasture and the high water mark (bare ground): (a) facing away from the wetland and (b) facing the wetland. 110 Chapter 5: Groundwater Quality and Hydrodynamics Figure 5.5: Design of the piezometer wells installed around Little Broadwater. Continuous monitoring of watertable depth was conducted with Odyssey Capacitive Water Level Recorders, which were installed in each piezometer. The loggers were programmed to record at hourly intervals and were powered by two 3.6V Lithium batteries. Water elevation data were downloaded every 2 months, and the probes were recalibrated every 2 to 4 months. The groundwater elevation was adjusted to m AHD using the surveyed elevations of the piezometers. Groundwater samples were obtained bi-monthly for physical and chemical analysis. The piezometers were initially pumped out at least 12 hours prior to sampling to ensure that stagnant ‘old’ water was not being sampled. An Amazon inline 12V submersible pump attached to 7 m flexible PVC pipe was used to pump groundwater to the surface. Sampling water was pumped into a jug that was rinsed three times with the sample water and then filled. This water was then used to rinse two 125 mL polyethylene bottles three times and fill the bottles. All air was excluded from the bottles before sealing and storing samples. After 111 Chapter 5: Groundwater Quality and Hydrodynamics collection, samples were stored at 4ºC as soon as possible, and frozen within 48 hours of collection. Samples remained frozen until laboratory analysis of soluble basic cations (calcium (Ca), magnesium (Mg), sodium (Na) and potassium (K)), acidic cations (Al, Fe and - 2- manganese (Mn)) and anions (Cl , SO4 ). Groundwater pH, EC and temperature were measured from the water remaining in the jug.