Relating Surface Water Alkalinity and Regional Geology Martha Conklin, Jean Morrill, Roger Bales, Ray Brice & Jonathon Whittier

Hydrology and Water ResourcesUniversity of ArizonaTucson, AZ

Alkalinity: Weathering of rocksCaCO3 + H+ Ca2+ + CO32- + H+ Ca2+ + HCO3-

QuestionsWhy 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

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 rocksLimestone is primarily composed of CaCO3.Dolomite is composed of CaMg(CO3)2

Most igneous and metamorphic rocks contain very little CaCO3, but instead are mostly less soluble silicates and metal (hydr)oxides.

ExpectationsRock Type

LIMESTONEOther MARINE SEDIMENTSCONTINENTAL SEDIMENTSIGNEOUS

Alkalinity

HIGHMODERATE TO HIGHMODERATELOW

Geology Classifications

GLOBE Alkalinity Sites

GLOBE & WQN Alkalinity Sites

State Average Alkalinity

Limestone and Dolomite

Other Marine Sedimentary

Continental Sedimentary

Igneous and Metamorphic

Average Alkalinty(mg/L as CaCO3)

WQN

Rock Type

Area (%)

Count

Alkalinity

Limestone

7.6

52

124

Other marine sedimentary

57.2

416

116

Continental sedimentary

13.7

44

169

Igneous and metamorphic

21.5

107

56

GLOBE

Rock Type

Area (%)

Count

Alkalinity

Limestone

7.6

3

205

Other marine sedimentary

57.2

20

109

Continental sedimentary

13.7

4

119

Igneous and metamorphic

21.5

8

32

Conclusions & Future ResearchThere are distinct geographical patterns for alkalinity in the US.GLOBE trends are similar to USGS trendsThere 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!)