Recharge systems for protecting and enhancing groundwate

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TOPIC 3
Modelling aspects and groundwater hydraulics
Gordon and Toze, 2003), and mixed human and animal wastes (Deng and Cliver, 1995; Nasser and Oman, 1999) have
a major role in the inactivation of microbial pathogens. In the case of Cryptosporidium, it has also been shown that
autochthonous microorganisms (and exo-enzymes from bacteria or fungi) in river water can influence the survival
of the oocytes (Medema et al., 1997), but there are relatively few investigations using groundwater (John, 2003).
The formulation of a computer model for determining minimum required retention times for microbial pathogen
die-off during aquifer storage is one piece of a complex puzzle. As managed aquifer recharge projects become
increasingly utilised, it is critical that water resource managers have a quantitative index of the risk of human exposure
to viable pathogenic microorganisms. This is one of the first studies to systematically investigate some of the
fundamental controls on the inactivation of microbial pathogens due to the activity of indigenous groundwater
bacteria with the aim of developing a reliable framework for predicting minimum aquifer retention times.
METHODS
There are a multitude of environmental stresses in the subsurface that could influence the survival of microbial
pathogens (Yates and Yates, 1988). In a recent survey of published decay rates for microbial pathogens, Toze (2004)
identified temperature, dissolved oxygen, water chemistry, source of water, as well as the type of pathogenic
microorganism, as the most commonly cited factors influencing pathogen survival. The viruses are more resistant to
pH and electrical conductivity (salt species and concentration) than bacteria, such that it has been postulated that
water chemistry directly impacts the activity of indigenous bacteria, which in turn has a secondary affect on the persistence
of viruses (Toze, 2004). As different studies of microbial survival have examined different pathogens under
a variety of environmental conditions, it is difficult to use published decay rates to formulate a predictive model.
The approach we are using is to systematically vary several environmental stresses identified by Gordon and Toze
(2003) as potential factors influencing the survival of enteric viruses: temperature, redox state and nutrient levels
are currently being considered (Sidhu et al., in press). Although treated effluent can have a range of nutrient types
and concentrations, we are first considering a complex organic carbon source and its effects on several pathogenic
microbes through the activity of non-pathogenic microorganisms native to groundwater.
Collection of data from pathogen survival experiments
Survival experiments are being undertaken to provide data to be used in the generation of the computer model as
well as using data from previous experiments. Current experiments are focusing on the influence of microbial activity
on the decay of a range of microbial pathogens. As it has been established that the activity of indigenous
groundwater microorganisms is the major influence on pathogen decay (Gordon and Toze, 2003), our focus is on
evaluating the influence of other biological, chemical and physical conditions on groundwater microbial activity
and the subsequent impact on the decay of enteric pathogens in groundwater. The biological processes affecting a
range of enteric pathogens that are commonly found in reclaimed water are being considered. The type of pathogen
has been shown to be an important factor as differences in decay rates have been observed between different
pathogens, even those that are genetically similar (Toze and Hanna, 2002; Gordon and Toze, 2003). As shown in
Figure 1, the survival of poliovirus and coxsackievirus under a similar set of environment stresses are quite different,
despite the microorganisms having genetic similarities. A list of some of the major pathogens to be investigated
is given in Table 1.
The major chemical and physical parameters of interest are the type of degradable organic carbon present, the redox
condition of the groundwater and a range of temperatures (5 - 20 °C). It is hypothesized that these parameters
impact on the metabolic activity and population structure of the indigenous groundwater microorganisms, thus
influencing the rate of decay of the different pathogens. The experiments on pathogen survival under the different
groundwater conditions are being undertaken using non-sterile groundwater obtained from a bore in the superficial
aquifer at the CSIRO laboratories in Perth, Western Australia. Groups of the selected pathogens are added to bio-
ISMAR 2005 ■ AQUIFER RECHARGE ■ 5th International Symposium ■ 10 –16 June 2005, Berlin