1.2 Planteamiento del problema
2.2.5 Capacidades coordinativas
2.2.7.2. Principio biológicos
As discussed in Section 6.1, there are several facilities operated by the Water Corporation that infiltrate a combined annual total of 7.23 GL of treated wastewater via infiltration basins on the Swan Coastal Plain. The disposal of wastewater at the Yanchep, Gordon Road, Halls Head and Caddadup WWTPs is principally in Tamala Limestone as the basins are lined with only a relatively thin layer of sand (Figure 6.10; Smith et al. 2012).
Figure 6.10 Locations of treated wastewater infiltration basins in Tamala Limestone. Reprinted from “)
Geohydrology of the Tamala Limestone Formation in the Perth Region: origin and role of secondary porosity,” by Smith et al. (2012), CSIRO Water for a Healthy Country National Research Flagship Report. Copyright 2012 by CSIRO. Reprinted with permission.
One of the first projects to document MAR in Tamala Limestone was conducted by Toze et al. (2002; 2004). It was a pilot indirect reuse scheme for the reuse of treated wastewater from the Halls Head WWTP for irrigation of public open space and road verges. Two recovery bores (SPB1 and SPB2) were installed at 80 m and 100 m from the infiltration ponds to abstract a mixture of ambient groundwater mixed with treated wastewater (Figure 6.11). This recycled water was then pumped to a storage tank for irrigation purposes. The results from the project showed improvements in recycled water quality (chemically and
microbiologically) compared with the treated wastewater and that the recycled water is more suitable for irrigation than native groundwater (Toze et al. 2004).
To further investigate infiltration of wastewater at the Halls Head WWTP, a pair of infiltration galleries was installed in the Tamala Limestone, less than 200 m from the infiltration basins (Figure 6.11). The Halls Head galleries were intended to operate for several years beginning in 2005, but due to redevelopment at the WWTP, the study was terminated after only 22 months. A total of 8.5 ML of treated wastewater was infiltrated. The infiltration galleries received a daily supply of treated wastewater. Due to low flows of wastewater at various times at the WWTP (typically at night), the pump delivering wastewater to the galleries would frequently stop and require manual resetting. As there was no full time operational staff and the galleries were not monitored daily, a timer was installed to automatically shut off the pump at night.
The watertable depth below the Halls Head galleries was relatively shallow (2 m below ground on average). Groundwater levels in bore 2/84 (southeast of the infiltration galleries) mainly responded to tidal
fluctuations, whereas groundwater levels recorded beneath the galleries were up to 10 cm higher and responded to both tidal fluctuations and the daily pulse of treated wastewater (Figure 6.12; Bekele et al. 2009). The delineation of groundwater flow directions in the aquifer was difficult due to the high
transmissivity of the Tamala Limestone. Figure 6.11 shows a schematic model of the capture zones for two bores SPB1 and SPB2, which each pump at roughly 217 kL/day to recover water infiltrating below the ponds (Toze et al. 2002). There was presumably a watertable mound below the ponds that produced flow radially outward and towards the galleries, but the shape of the mound and flow directions could not be confirmed.
The chemical composition of groundwater was interpreted in relation to mixing with seawater within the aquifer and the impact of recharge from the adjacent wastewater ponds (Bekele et al. 2009).
Figure 6.11 Map view of the infiltration galleries site at Halls Head relative to the ponds, monitoring bores and capture zones for recovery bores SPB1 and SPB2. Adapted from “Halls Head indirect treated wastewater reuse scheme,” by Toze et al. (2002), Client report to the Water Corporation. Copyright 2002. Adapted with permission.
Figure 6.12 Groundwater response to MAR using infiltration galleries at the Halls Head WWTP (Bekele et al. 2009). The locations of bores HH_E2 and 2/84 are shown in Figure 6.11 relative to the sites of infiltration. Reprinted from “Design and operation of infiltration galleries and water quality guidelines, Chapter 1. In: Toze S, Bekele E (eds), Determining the requirements for managed aquifer recharge in Western Australia,” by Bekele et al. (2009), Water Foundation Report. Copyright 2009. Reprinted with permission.
Indian
Ocean
SPB2 SPB1 Infiltration galleries site Infiltration pondsNorth
6/88 2/84 7/88 1/84 1/83 (approx. location) HH_E2 Inset 1 Period of effluent inflow Nightly shutoff Nightly shutoff Period of effluent inflow Period of effluent inflowNightly shutoff followed by failure of the supply pump to automatically restart
Period of effluent inflow
Preferential flow in the Tamala Limestone is a concern for MAR. As documented in a study of the hydrogeology of the Tamala Limestone (Smith et al. 2012), concerns have been raised about there being areas of cavern development and large-scale conduct flow in the Perth region, which could pose risks for MAR. There may be insufficient time for biodegradation to occur before the recycled water is abstracted for water supply or intercepted down-gradient for environmental benefits. However based on an extensive collection and analysis of considerable data for the Tamala Limestone, Smith et al. (2012) found that an appropriate conceptual model is one of dispersive flow through the formation pore system, rather than through large scale conduits, except where cavern development is known to be prevalent (e.g. Yanchep caves).
Bekele et al. (2014) conducted a MAR experiment and assessed aquifer travel times for treated wastewater in Tamala Limestone at the Floreat Infiltration Galleries site. Results from a three-dimensional solute transport model of the Tamala Limestone were compared with other tracer data. The study shows the limitation of relying on a single tracer to resolve residence times in the Tamala Limestone, and that heterogeneity can have a major influence on migration directions for recycled water plumes (Bekele et al. 2014).