Relating Surface Water Alkalinity and Regional Geology Martha Conklin, Jean Morrill, Roger Bales, Ray Brice & Jonathon Whittier Hydrology and Water Resources University of Arizona Tucson, AZ

Alkalinity: Weathering of rocks CaCO 3 Ca 2+ CO 3 2- HCO 3 - H2OH2O H+H+ CaCO 3 + H +  Ca 2+ + CO H +  Ca 2+ + HCO 3 -

Questions Why do GLOBE schools report a lower average alkalinity that non-GLOBE sites over the United States? Does regional geology explain the differences in alkalinity values?

GLOBE & USGS Data Comparison % of Sites with average  Value Average Alkalinity (mg/L as CaCO 3 )

Why is GLOBE mean alkalinity lower than WQN mean for United States? Distribution of GLOBE schools. Stream size: GLOBE samples streams; USGS samples rivers. Local watershed geology influences water alkalinity.

Why is alkalinity related to bedrock geology? Marine and continental sedimentary rocks Limestone is primarily composed of CaCO 3. Dolomite is composed of CaMg(CO 3 ) 2 Most igneous and metamorphic rocks contain very little CaCO 3, but instead are mostly less soluble silicates and metal (hydr)oxides.

Rock Type LIMESTONE Other MARINE SEDIMENTS CONTINENTAL SEDIMENTS IGNEOUS Alkalinity HIGH MODERATE TO HIGH MODERATE LOW Expectations

Geology Classifications Continental Sedimentary Other Marine Sedimentary Limestone Igneous and Metamorphic

GLOBE Alkalinity Sites mg/L as CaCO 3

GLOBE & WQN Alkalinity Sites

State Average Alkalinity mg/L as CaCO 3

Limestone and Dolomite mg/L as CaCO 3

Other Marine Sedimentary mg/L as CaCO 3

Continental Sedimentary mg/L as CaCO 3

Igneous and Metamorphic mg/L as CaCO 3

Average Alkalinty (mg/L as CaCO 3 )

Conclusions & Future Research There are distinct geographical patterns for alkalinity in the US. GLOBE trends are similar to USGS trends There is good agreement with bedrock; can we improve it by looking at soils? Expand rock categories to incorporate more detailed information on carbonate content. Expand analysis to other countries (as GLOBE schools help us find bedrock & soil data!)