Chemistry of Rivers and Lakes

Transcription

1 Chemistry of Rivers and Lakes Environmental Geochemistry DM Sherman, University of Bristol The Hydrologic Cycle 1

2 Chemistry of Rivers Controls on River Chemistry Rivers and Lakes are a combination of groundwater discharge and atmospheric precipitation. Hence, they tend to be more dilute and unsaturated compared to groundwater. Dissolved components in rivers derived from: Precipitation (rain) to give Cl -, SO 4-2 (pollution) Source rock weathering Organic matter from soil. Evaporation and Precipitation (e.g., of CaCO 3 ) 2

3 Chemistry of Major Rivers (mg/kg) Negro Lower Amazon Niger Yangtze Miss. Nile Yellow Colorado Na Mg SiO K Ca Cl SO HCO ph z (meq) Chemical Nature of Major Rivers The model of Gibbs (1970) divides rivers into three classes: precipitation, rock weathering and evaporation. Rainfall has high Na/(Na +Ca) and low TDS. As weathering increases, rocks release Ca due to carbonate dissolution and increase TDS. With evaporation, CaCO 3 ppts to increase Na/(Na+Ca) while TDS increases. 3

4 Chemical Nature of Major Rivers Stollard and Edmunds (1981) classification scheme emphasizes the importance of rock-weathering. Transport-Limited/Silicate Terranes TDS < 20 mg/l SmZ+ <200 µeq/l drain intensely weathered materials in transportlimited regime (e.g., Rio Negro). Enriched in SiO 2, Al, Fe, organic anions and have low ph. Significant feature is high (SiO 2 /SmZ+ > 2.4 (average crust = 2.4). Extreme weathering leaves residue of Fe and Al oxides, stripping alkali and alkaline earth cations plus silica. R. Negro Amazon 4

5 Weathering-Limited/Silicate Terranes TDS = mg/l Examples include Amazon, Congo. Weathering of silicates. Rate of erosion exceeds rate of chemical weathering. Cations leached from minerals in preference to SiO2 to give SiO2/SmZ+ = Availability of fresh rock results in higher SmZ+ and TDS than in transport-limited regimes. Carbonate Terranes TDS = mg/l Most major rivers (e.g., Mississippi). Dissolved ions derived from weathering of carbonates and evaporite minerals (halite and gypsum). SmZ+ between µeq/l have low Na+/(Na++Ca2+) and high Ca, Mg, HCO3-, SO4 (from oxidation of pyrite in reduced shales). Rivers tend to have 1:1 ratios of Na to Cl and (Mg+Ca) to (HCO3 + SO4). 5

6 Evaporite Terranes TDS > 250 mg/l Examples include Colorado Weathering of sandstone/evaporite terranes High ionic strength (SmZ+ > 3000 µeq/ l) 1:1 ratios of Na to Cl (from halite) and (Mg+Ca) to (HCO 3 +SO 4 ) from MgSO 4 / CaCO 3 dissolution. SiO 2 /SmZ+ < 0.1. Pollution of Rivers Primary forms of pollution to Rivers are: Organic matter (sewage etc.) Excess Nutrients (P, and N) Pesticides Acid Mine drainage Source: US EPA 6

7 Nitrate/Phosphate Pollution of Rivers What is Acid Mine Drainage?" Pollution associated with abandoned mines resulting from the oxidation of FeS 2 (pyrite). Very acid streamwaters with mobilized heavy metals. Some AMD is natural (the mining industry prefers that it be called acid-rock drainage ) 7

8 The Rio Tinto (Spain)" SW Spain, mining activity for > 3000 years. Pyrite (FeS 2 )" Pyrite is the most common sulfide mineral and occurs associated with hydrothermal ore deposits and in coal derived from marine deposits. In pyrite the oxidation state of sulfur is -1 (S 2-2 ) 8

9 Acid Generation by S 2 2- Oxidation" Oxidation of S 2-2 generates H + FeS 2 (s) + (7/2)O 2 (g) + H 2 O è Fe SO 2-4 (aq) + 2H + (aq) Oxidation of S 2- does not generate H + CuFeS 2 (s) + 4O 2 (g) è Cu +2 + Fe SO 4 2- (aq) FeS(s) + 2O 2 (g) è Fe 2+ + SO 4 2- (aq) ZnS(s) + 2O 2 (g) è Zn 2+ + SO 4 2- (aq) Kinetics of Pyrite Oxidation by Oxygen" FeS 2 (s) + (7/2)O 2 (g) + H 2 O è Fe SO 4 2- (aq) + 2H + (aq) rate (moles/m 2 -s) = [O 2 ] 0.5 [H ] 0.11 At ph 2.0 in air-saturated conditions [O 2 ] = mol/ kg, t 1/2 = 16 years for 0.05m 2 /g. 9

10 Fe 2+ Oxidation facilitated by Bacteria" Thiobacillus ferrooxidans is the most important bacteria that facilitates the oxidation of pyrite by generating Fe 3+ 4Fe 2+ (aq) + O 2 + 4H + è 4Fe 3+ (aq) + 4H 2 O Thiobacillus ferrooxidans is acidophillic, autotropic and fixes its carbon from atmospheric CO 2. Kinetics of Pyrite Oxidation by Fe 3+ " FeS 2 (aq) + 14Fe 3+ (aq) + 8H 2 O(l) è 15Fe 2+ (aq) + 2SO 4 2- (aq) + 16H + (aq) rate (moles/m 2 -s) = [Fe 3+ ] 0.3 [Fe 2+ ] 0.47 [H ] 0.32 At ph 2.0 with [Fe] tot = 10-2 and [Fe 2+ ]/[Fe 3+ ] = 0.01, t 1/2 = 150 days for 0.05m 2 /g. 10

11 Pyrite Oxidation Summary" (a) FeS 2 (s) + (7/2)O 2 (g) + H 2 O è Fe SO 4 2- (aq) + 2H + (aq) Slow (b) 4Fe 2+ (aq) + O 2 + 4H + è 4Fe 3+ (aq) + 4H 2 O Via Fe 2+ oxidizing chemolithoautotrophs (c) FeS 2 (aq) + 14Fe 3+ (aq) + 8H 2 O è 15Fe 2+ (aq) + 2SO 4 2- (aq) + 16H + (aq) Fast, autocatalytic mechanism Pyrite/FeOOH Stability" (species in italics are aqueous complexes) 11

12 Precipitation of FeOOH" Iron(III) (hydr)oxides precipitate out from solution when ph > 3. Goethite (a-feooh) Schwertmannite (Fe 8 O 8 (OH) 6 SO 4 ) Evaporite Deposits of FeSO 4 " Under acidic conditions, abiotic oxidation of Fe 2+ is slow. In the Rio Tinto, FeSO 4. nh 2 O precipitates upon evaporation. 12

13 Acid Mine Waste and Heavy Metals" Acid leaching and oxidation of galena (PbS), sphalerite (ZnS) and chalcopyrite (CuFeS 2 ) will release metals into solution. Secondary sulfates of Zn and Cu are quite soluble at low ph. Secondary sulfates will limit the solubility of Pb: Anglesite: PbSO 4 (pk = 7.76) Beudantite: PbFe +3 (AsO 4 )(SO 4 )(OH) 6 Beaverite: Pb(Fe,Cu) 3 (SO 4 ) 2 (OH) 6 Chemistry of Lakes 13

14 Input/Output Fluxes to a Lake C lake Q out = C gw Q gw + C riv Q in + F sed F scav Thermal Stratification of Lakes Winter Summer Epilimnion Thermocline Hypolimnion 14

15 Biological Oxygen Demand BOD = the amount of oxygen that would be consumed if all organic matter was consumed by bacteria. Unpolluted water has BOD < 5 mg/l Trophic Status Oligotrophic: clear water, oxic throughout the year in the hypolimnion (P < 10 ppb). Mesotrophic: possible anoxic hypolimnia during summer (10 < P < 30 ppb).. Eutrophic: blue-green algae; anoxic hypolimnia during the summer, oxic throughout the year in the epilimnion (P > 30 ppb). 15

16 An Oligotrophic Lake Thousand Island Lake in the Sierra Nevada (USA) is a typical oligotrophic lake due to very low nutrient inputs. Thousand Island Lake (California) A Eutrophic Lake This lake in SW England is impacted by nutrients in runoff from agricultural cropland and nutrient discharge from a small municipal sewage treatment plant. Copyright Tom Sims, University of Delaware 16

17 With time, the decay of organic matter consume oxygen and give anoxic layers in lakes, ponds and rivers. Eutrophication and Lakes This effect can be accelerated if biolimiting nutrients (P and N) are added by pollution. Hall Lake, WA (Balistrieri et al., 1994) Hall Lake is in suburban Seattle WA (USA) and shows eutrophication with an anoxic hypolimnion in the summer. The anoxic state may be due to anthropogenic PO 4-2 inputs. 17

18 Hall Lake, WA (Balistrieri et al., 1994) Hall Lake, WA (Balistrieri et al., 1994) 18

19 Mercury in Lakes An Acidified Lake: Loch Grannoch" 19

20 Continental Saline Lakes Mono Lake (Eastern California) 20

21 Tufas are strange rock formations that form when Ca 2+ bearing groundwater from springs mix with the NaHCO 3 water in Mono Lake. Ca 2+ + HCO 3 - = CaCO 3 (calcite) + H + Mono Lake Tufas Copyright Bruce Molnia, Terra Photographics Copyright Larry Fellows, Arizona Geological Survey Dead Sea and Death Valley Total Salinity of 30 % MgCl 2 53%, KCl 37% and 8% NaCl Na +, Mg +2, SO 4-2, Cl - 21

22 Evolution of Continental Saline Waters Na +, Ca +2, Mg +2, HCO 3-, SO 4-2, Cl - Calcite (CaCO 3 ) precipitates 2[Ca] > Alkalinity Alkalinity > 2[Ca] Na +, Ca +2, Mg +2, SO 4-2, Cl - Na +, Mg +2, HCO 3-, SO 4-2, Cl - Gypsum (CaSO 4. 2H 2 O) pptn. Sepiolite (MgSi 3 O 6 (OH) 2 ) pptn. [Mg] < [SO 4-2 ] [Mg] > [SO 4-2 ] 2[Mg] > Alkalinity 2[Mg] < Alkalinity Na +, Ca +2, Mg +2, Cl - Dead Sea Na +, Mg +2, SO 4-2, Cl - Death Valley Na +, HCO 3-, SO 4-2, Cl - Mono Lake Summary Composition of river waters results from mineral dissolution during weathering and precipitation of minerals during evaporation. Primary pollution threats to rivers includes: -Acid mine drainage Primary pollution threats to lakes includes: - Excess nutrients (P and N) leading to eutrophication. - Acidification from acid rain 22