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Paradigm for Drinking Water Treatment Environmental Science & Technology is published
Lindsay ErinbyAnderson, the American Chemical Society. 1155 Sixteenth Wendy H. Krkosek, Amina K. Street N.W., Washington, DC 20036
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Stoddart, Benjamin F Trueman, and Graham A. Gagnon Environmental Science &
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Alum Dose
Lake Color
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ACS Paragon Plus 10 Environment 0 1999
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Environmental Science & Technology
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Lake Recovery Through Reduced Sulphate Deposition: A New Paradigm for Drinking Water Treatment
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Ms. Lindsay E. Anderson1, Dr. Wendy H. Krkošek2, Ms. Amina K. Stoddart1, Mr. Benjamin F.
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Trueman1 and Dr. Graham A. Gagnon1*
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Department of Civil & Resource Engineering
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Dalhousie University
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Office D-514, 1360 Barrington Street
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Halifax, Nova Scotia, Canada
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B3H 4R2
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Halifax Water
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Halifax, Nova Scotia, Canada
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B3P 2V3
450 Cowie Hill Road
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Submitted to Environmental Science and Technology for review and possible publication
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December 2016
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Word count: 6968
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Environmental Science & Technology
ABSTRACT
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This study examined sulphate deposition in Nova Scotia from 1999-2015, and its association
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with increased pH and organic matter in two protected surface water supplies (Pockwock Lake
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and Lake Major) located in Halifax, Nova Scotia. The study also examined the effect of lake
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water chemistry on drinking water treatment processes. Sulphate deposition in the region
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decreased by 68%, while pH increased by 0.1-0.4 units over the 16-year period. Average
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monthly color concentrations in Pockwock Lake and Lake Major increased by 1.7 and 3.8x,
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respectively. Accordingly, the coagulant demand increased by 1.5 and 3.8x for the water
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treatment plants supplied by Pockwock Lake and Lake Major. Not only was this coagulant
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increase costly for the utility, it also resulted in compromised filter performance, particularly for
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the direct-biofiltration plant supplied by Pockwock Lake that was found to already be operating
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at the upper limit of the recommended direct filtration thresholds for color, total organic carbon
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and coagulant dose. Additionally, in 2012-2013 geosmin occurred in Pockwock Lake, which
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could have been attributed to reduced sulphate deposition as increases in pH favor more diverse
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cyanobacteria populations. Overall, this study demonstrated the impact that ambient air quality
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can have on drinking water supplies.
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INTRODUCTION
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As a result of successful air emissions control, a number of studies have shown evidence of
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lake recovery from acidification.1-3 Lake recovery is defined by increasing acid neutralization
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capacity (ANC), alkalinity and/or pH,4 and is often associated with increasing natural organic
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matter (NOM) as measured by dissolved organic carbon (DOC). Increasing concentrations of
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DOC in surface waters in the Northern hemisphere have been reported in areas that were
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previously exposed to chronic sulphate (SO4) deposition through acid rain,5-8 and therefore
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increasing DOC is often viewed as evidence for recovery from acidification. For example,
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Monteith et al.8 studied the spatial distribution of DOC trends in data collected in six North
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European and North American countries between 1990 and 2004 and found widespread
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significant upward trends in DOC in the northeastern portion of the United States (US), southern
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Nordic regions, and in the United Kingdom (UK), which Monteith and co-workers related to
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reductions in atmospheric deposition of SO4 across large areas. Garmo et al.3 analyzed trends in
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surface water chemistry for 12 sites in Europe and North America and found that during the
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period between 1990 and 2008, sulphate deposition declined at 87% of study sites,
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corresponding with increasing DOC concentrations. Other instances of increasing DOC, pH and
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diversity have occurred in lakes without associated increases in nutrient levels, 9 leading to the
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concept that Nova Scotia lakes are undergoing some form of change, observed through increased
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NOM, which may be associated with lake recovery.
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In drinking water, NOM may contribute to taste, color, and odor issues that can be
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problematic from a consumer standpoint. As well, NOM from source water similar to the lakes
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studied have shown to contribute to the formation of chlorinated disinfection by-products (DBPs)
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following chlorination.10-12 Accordingly, coagulation processes are widely used to destabilize
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negatively charged particulate matter13 and through enhanced coagulation, conventional
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treatment processes can be adapted to remove NOM.14 It has been suggested that surface waters
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recovering from acidification produce NOM with a greater hydrophobic fraction as the solubility
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of NOM increases,15-17 which will have potential to increase coagulant demand. Such changes in
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influent water quality will also have implications for energy use and carbon emissions associated
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with water treatment processes. The operational embodied energy for a water treatment facility
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represents the energy needed to produce a unit volume of drinking water, and consists of direct
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and indirect energy. Direct energy is comprised of electricity and fuel consumption, and indirect
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energy is from the production and transport of water treatment chemicals. Santana et al.18
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showed that increases in chemical dosing at a water treatment plant in Florida was responsible
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for 14.5% of the total operational embodied energy for the facility. Accordingly, increases in
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NOM concentration and changes in composition associated with recovery from acidification are
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anticipated to have significant impact on water treatment, though there are limited, if any data
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reported in the literature.
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In addition to abiotic water quality indicators for recovery (e.g. increasing NOM
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concentration), changes in biotic water quality in terms of species richness and taxonomic
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composition are among the best-known indicators of biological recovery from acidification.19
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Experimental acidification of lakes has demonstrated loss of algal species,20, 21 reduced species
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richness, and changes in algal community structure as the phytoplankton community shifts from
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cyanobacteria to large dinoflagellates.21-24 Anthropogenic acidification has also caused long-term
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changes in algal community abundance, spatial distribution, and taxonomic composition.21-24
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Findlay et al.21 showed that experimental and anthropogenic acidification had an impact on the
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species diversity of phytoplankton, which was positively correlated with pH. Findlay et al.21 also
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noted that cyanobacteria were significantly reduced below pH 5.1, and increased during recovery
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at a pH between 5.5 and 5.8. In the northeastern US, decreased sulphate deposition throughout
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the 1980’s-2000’s and subsequent increases in ANC corresponded with increased phytoplankton
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species richness.25 It has been widely reported that some cyanobacteria produce taste and odor
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compounds in surface waters.26-28 For example, one of the most prevalent taste and odor
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compounds—geosmin (trans-1,10-dimethyl-trans-9-decalol) is produced by a subset of
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filamentous cyanobacteria and is a common challenge for drinking water utilities. In Fall 2012,
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previously unreported incidents of geosmin were identified in Pockwock Lake.29 Prior to 2012,
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geosmin was not a water quality concern for Halifax Water. Consumer complaints regarding the
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earthy, musty smell of geosmin in treated tap water began in 2012, which initialized Halifax
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Water’s geosmin monitoring routine.
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The objective of this study is to examine the impact of decreased sulphate deposition and its
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effect on lake water quality and water treatment practices over the period 1999-2015. This study
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focuses on two lakes that are used as drinking water supplies in Nova Scotia, Canada. It also
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contributes to understanding the impact of decreased sulphate deposition on water treatment
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practices and has implications to water utilities in regions with surface waters that have been
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impacted by anthropogenic emissions of sulfur dioxide (SO2) to the atmosphere.
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EXPERIMENTAL
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Study sites. Lake Major and Pockwock Lake are protected watershed areas and are not
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impacted by wastewater discharges. Both lakes are used for drinking water supply and are
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operated by Halifax Water located in the Halifax Regional Municipality of Nova Scotia, Canada.
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Pockwock Lake is characterized as a low-pH (pH
