Abstract
Historically, enteric disease-causing bacteria such as Cryptosporidium, Giardia, and E. coli have been the main organisms for drinking water treatment in the US. Through a
combination of impactful legislation in the form of the Safe Drinking Water Act (SWDA) in
1974, and advancements in water treatment technologies, we have made outbreaks from
these diseases a rare occurrence. In the wake of this, a new category of respiratory
waterborne illnesses has become more common. Public health agencies such as the Center
for Disease Control (CDC) have seen an uptake in cases of respiratory infections caused by
various opportunistic pathogens (OPs), including Legionella, non-tuberculous
Mycobacterium, and Pseudomonas. This category of pathogens is known for their chlorine
resistance and ability to survive and thrive in the biofilm of water distribution systems.
Currently, waterborne illness is thought to cause 7.15 million illnesses, 118,000 hospitalizations, and 6,630 deaths annually in the US, resulting in an estimated $3.33 billion
in direct healthcare expenses. These numbers are primarily driven by illness caused by the
aforementioned OPs. Typically, most waterborne pathogens are associated with the natural
environment since they are often found in soils and natural water bodies. Because of this, the
SWDA has strict treatment regulations for systems that utilize surface water. In contrast,
groundwater is assumed to be much less microbially active, so systems that utilize
underground aquifers have lower treatment standards.
Despite these assumptions, recent reporting from the CDC has estimated that 38% of waterborne disease outbreaks have been associated with groundwater sources. One potential
reason explaining this could be that biofilms found within a distribution system have the
potential to reintroduce pathogens that were not originally present in the source water. It is
possible that the lower treatment standards used by groundwater systems, particularly with
relation to lower chlorine residuals, can allow biofilms to flourish and increase the potential
risk of OPs. This study aims to investigate this issue by comparing two distinct drinking
water systems in geographically close Idaho cities, one being primarily served by the
Clearwater River, and the other sourcing drinking water from the Palouse basin aquifer.
The results of this study have indicated significant distinctions between the groundwater and surface water systems. First, we were able to confirm that many general assumptions
regarding source water differences were confirmed such as high dissolved oxygen levels, low
alkalinity levels, and high microbial activity levels. Additionally, we observed high levels of
pathogen presence in the surface water. Despite this, we also concluded that both water
systems studied were able to treat their respective water sources to similar standards. In both
systems, the water after treatment had very low levels of microbial activity and near zero
detection rates of potential pathogens.
These similarities end after the water is delivered to the distribution system. Moscow’s distribution system was found to have consistently lower chlorine residuals, often reaching
negligible levels throughout many of the sampling points. This not only led to higher
microbial activity, but it also led to higher detection rates of OPs as compared to the
Lewiston system. We also found that Moscow operated at different chlorine levels depending
on the season, aiming for a higher chlorine residual during the summer months due to a
higher perceived risk of pathogens. The results in this change of management strategy were
counter-intuitive, showing that the system actually had a significantly reduced risk in
opportunistic pathogens throughout the summer months as compared to the winter.
The results of this study show that groundwater-sourced drinking water systems are still susceptible to waterborne pathogens, similar to the concern with surface water-served
systems. The main finding is that this risk may come from the differing strategies in chlorine
residual management presented by the two systems. Particularly that a low chlorine residual,
as often observed in the groundwater system, can cause the establishment of biofilm
throughout a system. This biofilm can then cause microbial regrowth and pathogen
reintroduction despite the quality of the source water. Furthermore, this study also suggests
that OPs are more common in groundwater sourced systems compared to surface water.
Indicating that the concern regarding the increase in respiratory drinking water disease
should extend to groundwater systems in addition to surface water sourced systems.