5/6) Phosphorus, metals, and salt

details on phosphorus, and as a contaminant

essential nutrient, often the limiting factor, excess P leads to eutrophication

particulate phosphorus sticks to particles, not particularly useful for microorganisms

what are potential sources of phosphorus?

urbanization

effluents from wastewater treatment plants

agricultural runoff

upstream sources

where is phosphorus more of an issue, surface or groundwater?

surface water

sediments can contain phosphate attached to sediments, but phosphorus measured in river is low

what does phosphorus easily attach to?

iron and aluminum oxides and hydroxides

how is phosphorus released from iron oxides

in reducing conditions, iron gets reduced, becoming soluble and iron oxide dissolves

any phosphate that was stuck to it has nothing to attach to anymore, it gets released

true or false: phosphorus contamination can be buffered by wetland

true: more organic life present, so more phosphorus use, it will be less intense of a contaminant

details on manganese in alberta groundwater including MAC and sources

MAC = 0.12 mg/L
aesthetic objective = <0.02 mg/L

reducing component, likes to hang out in water

rocks from marine shales, weathering, deposit, places where pyrite oxidation happens

will exceed MAC in post-oxic zone

details on iron in alberta groundwater

no MAC
aesthetic objective = <0.3 mg/L

taste of water affected

oxidized Fe3+ is usually in solids as Fe(III)
reduced Fe2+ is usually dissolved in water

where will iron exceed AO?

will exceed in post oxic zone, it gets reduced

will not exceed in oxic zone, iron would immediately precipitate
will not in sulfidic zone, would precipitate as iron sulphate immediately

details on acid groundwater

oxidation of pyrite

one of the most strongly acid-producing processes in nature

lowering of groundwater table may expose and oxidize pyrite
if pyrite remains reduced in subsurface, then this wouldn’t happen

what are consequences of acid groundwater

acid water seeps out, metals can be too high, or just the effects of acid

pH<3

releases high amounts of Fe 3+ and SO42-

if pH is less than 4, Fe3+ becomes soluble

other heavy metals soluble at low pH such as Zn and Cu
as pH increases to >4, Cu adsorbs and concentrations decrease rapidly. zinc is the same but at a higher pH

what happens to pyrite oxidation in a closed system?

which system would result in lower pH values?

it stops because you can’t resupply pyrite to continue the reaction

open system results in lower pH values because you can replenish the reactants and continue the reaction

what’s important about arsenic in groundwater

oxidation of arsenopyrite liberates arsenic

shallow groundwater may have significantly elevated concs of arsenic

definition of heavy metals and metalloids, examples, extra details

heavy metal: any metallic chemical element that has a relatively high density (>5 g/cm3) and is toxic or poisonous at low concentrations

metalloid: has both metal and nonmetal properties

ex: As, Cd, Cr, Hg, Pb. arsenic, cadmium, chromium, mercury, lead

many metal(loids) are redox-active, and thus are mobilized or immobilized via redox processes

where would you expect to see the highest trace metal concentrations in groundwater

heavy metal and trace elements can be found at any depth

concs are dependent on:
natural presence of metal in aquifer sediments
groundwater chemistry (oxic vs anoxic)
potential anthropogenic sources

what are common anthropogenic sources of arsenic?

insecticides, weed killers, wood presevatives

added to animal feed to prevent diseases

pigments/paint

manufacturing of electronic components

medicine

chemical warfare

what are sources of arsenic contamination in water?

natural/human activities

arsenic is an element that occurs naturally

volcanic activity, erosion of rocks and minerals, and forest fires

agricultural applications, mining and smelting

90% of arsenic used in industry is for wood preservative purposes

what is the MAC of arsenic

0.001 ppm / 10 ppb

Canada: as low as reasonably achievable (ALAR)
US: maximum contaminant level goal (MCLG) = 0ppm

before 2001 = 50 ppb

which form of arsenic is more mobile?

As3+ more mobile than As5+

As5+ is oxidized, it sorbs easily on iron hydroxides

As3+ is reduced, is more mobile

arsenic mobility is linked closely with solid Fe oxides and hydroxides

what zones will As exceed the MAC?

very few samples exceed

middle of post-oxic zone has highest As conc. about half the upper 50% are above the MAC

if there is lots of iron in GW, what form is it in?

it must be Fe2+ because GW is typically pH 6-8, meanings Fe is under reducing conditions

what form of iron causes staining?

what form of iron causes bas taste?

Fe3+ causes staining
at the surface, there is lots of oxygen, so Fe2+ will oxidize to Fe3+

Fe2+ causes bad taste

what are the problems with arsenic in groundwater in SE asia?

1). As concentrations are patchy, there are general trends but it’s difficult to predict where you might find high arsenic wells

2). weathering of himalayas, if that material has arsenopyrite, then the arsenopyrite is deposited in the floodplains
floodplains have lots of organic-rich sediments and fine-grained materials
^these conditions are suitable for slow moving water, reducing down the redox ladder

where are arsenic concentrations lowest? where is it highest?

lowest at the surface (near-surface GW and surface water) AND in the deepest wells

highest in mid-depth wells (from 10-50m)

explain the relationship between arsenic and iron in sediments

if there is lots of iron in water, you get lots of As

in oxidizing conditions nothing happens, but in reducing conditions, the As gets released

shallow wells: when iron oxides exist, there is enough oxidizing power so that As sticks to iron

middepth wells: more reducing environment and iron reduces, liberating the As from the compound and releasing into the water

explain the sequence of events for As-Fe relationship

first need a source of As, then As in the aquifer sorbs to iron oxides under aerobic conditions

when anaerobic conditions evolve, reductive dissolution of iron oxides may occur

Fe-As compound dissolves, liberating As from solid materials to the groundwater, turning it from As5+ to the more mobile As3+

degradable organic carbon is needed to fuel redox reactions

what conditions are needed for arsenic contamination to occur

arsenic rich materials eroded and weathered in a specific way

areas with active mountain belts and associated sedimentary and deltaic basins

what type of rocks are a major geologic source of arsenic?

slates and phyllites

what is the problem with private wells?

public wells are regularly tested for various contaminants including arsenic

private wells it’s up to the owner to make sure it’s okay to drink

usually private owners only test their water after something goes wrong (bad taste/smell/colour, or a spill nearby)

arsenic has no taste, smell, or colour, and can do a lot of damage in small concs

it also costs money to test water, and even more to remove contaminants

what are ways to remove arsenic from drinking water?

what are ways that don’t work to remove arsenic from water?

reverse osmosis and distillation

reverse osmosis is cheaper and takes less time and energy

however most home rev osmosis systems can’t reduce arsenic concs more than 15%

doesn’t work: water softeners, carbon filters, UV treatment

what is the MAC for fluoride?

what are potential health effects?

what is recommended conc for fluoride?

1.5 mg/L

more than that = mottled teeth, bone damage

0.7 ppm

less than recommended also unhealthy for teeth

what are geologic sources of fluoride?

apatite: want to get phosphate out of apatite, end up liberating fluoride
fluorite
cryolite: lower melting point of ores that we want to get aluminum out of
micas
amphiboles
sellaite

anthropogenic: apatite (fertilizer) and cryolite (aluminum factories) mostly

why do all the data points plot under the line in this figure? explain more about it

they are subsaturated with respect to fluorite

if you had so much Ca and F that water plots above, the mineral fluoride would precipitate out until you were back at the black line

on the line is in eqlm with respect to the mineral

at high Ca concs, you’ll get relatively low F concs, because if you had more F, the mineral would precipitate out above the line

what do water types have in common in alberta groundwater

dominated by sodium, don’t have much calcium

calcium bicarbonate water can exchange cations with clay minerals to get sodium back, sodium bicarbonate rich

how can we remediate high fluoride in water?

adding Ca2+ (gypsum)

increasing Ca2+ puts you at a point on this graph where the eqlm line is lower, so fluorite will precipitate out and F decreases

gypsum is far more soluble than fluorite

details on chromium

naturally occurring in serpentinitesm
anthropogenically used for chrome plating, dyes, wood preservation

highly soluble at low pH

adsorbs to solids when pH is moderate-high

Cr3+ is more reducing, insoluble
Cr6+ is more mobile

chromate will adsorb to Fe/Al oxides in soils

MAC is 0.05 mg/L
De minimis = keep as low as you can even if its below MAC

details on cadmium

naturally found deep in subsurface in ores, released during volcanic activity

in solution occurs as Cd2+

solubility similar to iron, high at low pH, low at high pH

high adsorption potential in soils

MAC is 0.0007 mg/L

Cd bioaccumulates/magnifies

details on lead

common source is lead-acid batteries, historically in gasoline

occurs as Pb2+

MAC is 0.005mg/L

strong adsorption by soil at pH >5

what is the general rule for solubility of metals in water

high solubility at low pH
low solubility at neutral+ pH

high potential for adsorption at elevated pH values if there are sorbing materials available

extra details for sorption of metals in water

what metal is the first to adsorb and why?

pH values below 3 and they all remain in solution

Cr is the first to adsorb: high charge and small size. more affinity to negative compounds

order is charge related then size

what are sources of salts

naturally occurring, but are often anthropogenically enhanced like from overpumping aquifers near saltwater causing an intrusion

de-icing roads, saline spills

karstic environment: water on one side comes in, karstic surfaces allow the saline water to get to the surface where the evaporite is not on top of that side. salt is removed from evaporite formation

what are problems with salt contamination

crop damage (saline soils are generally unfit for plant growth)

weathering pipelines