Editing IPCC:AR6/SROCC/Chapter-2 (section)

==== 2.3.1.2 Water Quality ====

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Glacier decline can influence water quality by accelerating the release of stored anthropogenic legacy pollutants, with impacts to downstream ecosystem services. These legacy pollutants notably include persistent organic pollutants (POPs), particularly polychlorinated biphenyls (PCBs) and dichlorodiphenyl-trichloroethane (DDT), polycyclic aromatic hydrocarbons, and heavy metals (Hodson, 2014 <sup>[[#fn:r282|282]]</sup> ) and are associated with the deposition and release of black carbon. There is ''limited evidence'' that some of these pollutants found in surface waters in the Gangetic Plain during the dry season originate from Himalayan glaciers (Sharma et al., 2015 <sup>[[#fn:r283|283]]</sup> ), and glaciers in the European Alps store the largest known quantity of POPs in the Northern Hemisphere (Milner et al., 2017 <sup>[[#fn:r284|284]]</sup> ). Although their use has declined or ceased worldwide, PCBs have been detected in runoff from glacier melt due to the lag time of release from glaciers (Li et al., 2017 <sup>[[#fn:r285|285]]</sup> ). Glaciers also represent the most unstable stores of DDT in European and other mountain areas flanking large urban centres and glacier derived DDT is still accumulating in lake sediments downstream from glaciers (Bogdal et al., 2010 <sup>[[#fn:r286|286]]</sup> ). However, bioflocculation (the aggregation of dispersed organic particles by the action of organisms) can increase the residence time of these contaminants stored in glaciers thereby reducing their overall toxicity to freshwater ecosystems (Langford et al., 2010 <sup>[[#fn:r287|287]]</sup> ). Overall the effect on freshwater ecosystems of these contaminants is estimated to be low ( ''medium confidence'' ) (Milner et al., 2017 <sup>[[#fn:r288|288]]</sup> ).

Of the heavy metals, mercury is of particular concern and an estimated 2.5 tonnes has been released by glaciers to downstream ecosystems across the Tibetan Plateau over the last 40 years (Zhang et al., 2012 <sup>[[#fn:r289|289]]</sup> ). Mercury in glacial silt, originating from grinding of rocks as the glacier flows over them, can be as large or larger than the mercury flux from melting ice due to anthropogenic sources deposited on the glacier (Zdanowicz et al., 2013 <sup>[[#fn:r290|290]]</sup> ). Both glacier erosion and atmospheric deposition contributed to the high rates of total mercury export found in a glacierised watershed in coastal Alaska (Vermilyea et al., 2017 <sup>[[#fn:r291|291]]</sup> ) and mercury output is predicted to increase in glacierised mountain catchments (Sun et al., 2017 <sup>[[#fn:r292|292]]</sup> ; Sun et al., 2018b <sup>[[#fn:r293|293]]</sup> ) ( ''medium confidence'' ). However, a key issue is how much of this glacier-derived mercury, largely in the particulate form, is converted to toxic methyl mercury downstream. Methyl mercury can be incorporated into aquatic food webs in glacier streams (Nagorski et al., 2014 <sup>[[#fn:r294|294]]</sup> ) and bio-magnify up the food chain (Lavoie et al., 2013 <sup>[[#fn:r295|295]]</sup> ). Water originating from rock glaciers can also contribute other heavy metals that exceed guideline values for drinking water quality (Thies et al., 2013 <sup>[[#fn:r296|296]]</sup> ). In addition, permafrost degradation can enhance the release of other trace elements (e.g., aluminium, manganese and nickel) (Colombo et al., 2018 <sup>[[#fn:r297|297]]</sup> ). Indeed, projections indicate that all scenarios of future climate change will enhance the mobilisation of metals in metamorphic mountain catchments (Zaharescu et al., 2016 <sup>[[#fn:r298|298]]</sup> ). The release of toxic contaminants, particularly where glacial melt waters are used for irrigation and drinking water in the Himalayas and the Andes, is potentially harmful to human health both now and in the future (Hodson, 2014 <sup>[[#fn:r299|299]]</sup> ) ( ''medium confidence)'' .

Soluble reactive phosphorus concentrations in rivers downstream of glaciers are predicted to decrease with declining glacier coverage (Hood et al., 2009 <sup>[[#fn:r300|300]]</sup> ) as a large percentage is associated with glacier-derived suspended sediment (Hawkings et al., 2016 <sup>[[#fn:r301|301]]</sup> ). In contrast, dissolved organic carbon (DOC), dissolved inorganic nitrogen and dissolved organic nitrogen concentrations in pro-glacial rivers is projected to increase this century due to glacier shrinkage (Hood et al., 2015 <sup>[[#fn:r302|302]]</sup> ; Milner et al., 2017 <sup>[[#fn:r303|303]]</sup> ) ( ''robust evidence, medium agreement)'' . Globally, mountain glaciers are estimated to release about 0.8 Tera g yr -1 (Li et al., 2018 <sup>[[#fn:r304|304]]</sup> ) of highly bioavailable DOC that may be incorporated into downstream food webs (Fellman et al., 2015 <sup>[[#fn:r305|305]]</sup> ; Hood et al., 2015 <sup>[[#fn:r306|306]]</sup> ). Loss rates of DOC from glaciers in the high mountains of the Tibetan Plateau were estimated to be ∼ 0.19 Tera g C yr -1 , (Li et al., 2018 <sup>[[#fn:r307|307]]</sup> ) higher than other regions suggesting that DOC is released more efficiently from Asian mountain glaciers (Liu et al., 2016 <sup>[[#fn:r308|308]]</sup> ). Glacier DOC losses are expected to accelerate as they shrink, leading to a cumulative annual loss of roughly 15 Tera g C yr -1  of glacial DOC by 2050 from melting glaciers and ice sheets (Hood et al., 2015 <sup>[[#fn:r309|309]]</sup> ). Permafrost degradation is also a major and increasing source of bioavailable DOC (Abbott et al., 2014 <sup>[[#fn:r310|310]]</sup> ; Aiken et al., 2014 <sup>[[#fn:r311|311]]</sup> ). Major ions calcium, magnesium, sulphate and nitrate (Colombo et al., 2018 <sup>[[#fn:r312|312]]</sup> ) are also released by permafrost degradation as well as acid drainage leaching into alpine lakes (Ilyashuk et al., 2018 <sup>[[#fn:r313|313]]</sup> ).

Increasing water temperature has been reported in some high mountain streams (e.g., Groll et al., 2015; Isaak et al., 2016 <sup>[[#fn:r314|314]]</sup> ) due to decreases in glacial runoff, producing changes in water quality and species richness (Section 2.3.3). In contrast, water temperature in regions with extensive glacier cover are expected to show a transient decline, due to an enhanced cooling effect from increased glacial melt water (Fellman et al., 2014 <sup>[[#fn:r315|315]]</sup> ).

In summary, changes in the mountain cryosphere will cause significant shifts in downstream nutrients (DOC, nitrogen, phosphorus) and influence water quality through increases in heavy metals, particularly mercury, and other legacy contaminants ( ''medium evidence, high agreement'' ) posing a potential threat to human health. These threats are more focused where glaciers are subject to substantial pollutant loads such as High Mountain Asia and Europe, rather than areas like Alaska and Canada.

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