Water chemistry overview4 Oct 2001

• Announcements– Canoe trip!! – Exam next Wednesday– College certified drivers?

• Today's lecture– The idea of budgets– Factors influencing chemical composition of

freshwater– Some important ions

Budgets

Surface Inflow

Wind-blown particles Evaporation

Surface Outflow

Groundwater discharge

Groundwater recharge

Sedimentation

Precipitation

Internal cycling

Internal cycling driven by

• Seasonal changes in biological demand

• Food web dynamics• Seasonal changes in the

chemical environment (usually oxygenated/not oxygenated)

• Seasonal weather changes

• Availability of limiting nutrients

Internal cycling

(Aquatic) Biogeochemistry

Biogeochemistry is chemistry in an environmental context that considers the biology, geology, and chemical interactions of elements.

Biogeochemistry focuses on the cycling of substances between living (biotic) and non-living (abiotic) components of ecosystems.

(see Schlesinger, W.H. 1997 Biochemistry for more information about biogeochemistry)

Salinity

Salinity: summed concentrations of the major cations (+) and anions (-) in the water (mg/liter or meq/liter)

Major ions = concentrations > 1mg/literConductivity (specific conductance): surrogate

measure of salinity, measures the ease by which an electrical current passes between two electrodes submersed in the water (microSiemens (S)/cm at 25C)

Total dissolved solids (TDS): dry a filter of the water sample; TDS= change in weight of filter containing "stuff" in the water. (not accurate)

Factors determining the occurrence of an element

1. Occurrence and forms of elements in water

1. Solubility

2. Disassociation

2. Major source pools

1. Atmosphere (dry/wet deposition, etc)

2. Lithosphere (rocks)

3. Biosphere (humans…)

3. Biological demand (e.g. limiting elements)

Liebig's Law of the Minimum:

C H O P K N

Environment 100 50 50 5 10 25

Organism 50 25 25 5 1 20

Lithosphere contribution to major ions

Other terms

• "Soft water" low salinity (~conductivity) often in drainages with acidic igneous rocks (e.g. granite)

• "hard water" high salinity, due to large concentrations of carbonate (CO3

=), usually from the the drainage of calcareous deposits (e.g., limestone)

Proportional occurrence controlled by:

• Types of minerals in the area

• Atmospheric inputs

• Evaporation vs precipitation

Ion concentration (salinity) lake space

• Note: simplification that works best at global scale

• At small spatial scales, precipitation, evaporation, and climate are similar, so differences in geology predominate

Sal

inity

(T

DS

) (m

g/l)

Dominant ions

Pre

cipi

tatio

n or

run

off

Low

Hig

h

CaHCO3 NaCl

Dominant ions

• On average…

Ca2+ > Mg2+ > Na+ > K+

HCO3- > SO4

2- > Cl-

• Mostly rock-weathering

• In rich agricultural systems, NO3- can be

important (> 1mg/liter)• P,N,Fe and trace elements (micronutrients) are

biologically important (although unimportant in terms of total ions)

Exceptions• Ocean-derived spray and rain increase Na+ > Mg2+

• Rock-weathering sequence is less applicable to small drainages (local precipitation)

• Acidification leads to H+ > Al ions > Ca2+ > Mg2+

• Local applications of road salt (NaCl) and fertilizers increase SO4

-, Cl-, NO3- and decrease

HCO3-

• Inter-year variation and climatic signals (= changes in evaporation rates or precipitation)

Changes in ions

• Conservative ions: little change over time (Mg2+, Na+, K+)

• Dynamic ions: concentrations are influenced by organism metabolism (HCO3

-, SO4-, Si, Fe, Ca2+,

P, N) ==> Biological demand

Calcium (Ca2+)

• From rocks• Critical for membrane ion exchange• Can limit the distribution of organisms that use Ca2+

in building exoskeletons or shells

(e.g. crayfish uncommon at [Ca2+]< 2 ppm)

• Interacts with CO3- (carbonate) to form CaCO3 (=

whiting or marl) in the presence of high levels of photosynthesis

(loss of CO2)

Annual Calcium distributions

Silica

• From rocks (igneous)• Found in many forms• High biological demand, primarily from diatoms (to build

frustule), also sponges and some macrophytes

Diatom (algae) frustules

Distribution of silica (Si) under stratified conditions

Annual Silica

Oligotrophichardwaterlake

Given the above isopeth plot of calcium distributions in Wintergreen Lake, 1. plot the Ca conc. vs. depth profile for mid-August2. relative to that Ca profile, sketch in plausible temperature and oxygen profiles (say max

O2 is 8 mg/l and max temp is 25 oC)3. give a plausible explanation(s) as to the mechanisms that increase calcium

concentration in the water column during the winter months.

Question of the day: