Department of Earth Sciences

Department of Earth Sciences

Long-Term Average Mineral Weathering Rates from Watershed Geochemical Mass

Balance Methods: Using Mineral Modal Abundances to Solve More Equations in More Unknowns

Jason R. Price,1,* Noel Heitmann2, Jennifer Hull1, and David Szymanski3 1Dept. of Earth Sciences, P.O. Box 1002, Millersville University, Millersville, PA 17551

Abstract

The number of phases for which weathering rates can be determined by watershed geochemical mass balance is limited by the number of equations that can be constructed from elemental flux losses from the watershed and mineral stoichiometries. Mass balance studies of watershed weathering rates routinely use the flux losses of the six major cations SiO2, Al, Na, K, Mg, and Ca. Analyses of these species in water are common, but following matrix algebraic methods limits the number of weathering rates that can be calculated to six.For the Brubaker Run watershed located in the northern Piedmont Physiographic Province of Pennsylvania (USA), long-term (103-106 year) watershed chemical flux losses havebeen determined using 10Be-derived total denudation rates and zirconium-normalized total chemical concentrations from bedrock and soils. Chemical flux losses calculated from solidphase data have three advantages: They (1) Permit generation of a relatively large number of equations because both major and trace analyses are included; (2) eliminate the need for many years of regular (e.g., weekly) sampling and chemical analyses of stream water and atmospheric precipitation, and measurement of hydrologic parameters (i.e., precipitation, stream discharge, etc.); and (3) long-term weathering rate calculations need not address biomass.

For Brubaker Run, eight minerals are involved in weathering; the five primary minerals are REE-rich epidote, ankerite, almandine-spessartine garnet, muscovite, chlorite, and the three secondary products are weathered muscovite, kaolinite, and gibbsite. The long-term average weathering rates of these minerals were calculated using the six major cations, and two trace elements selected from Rb, Sr, Ba, La, Pr, Nd, Sm, Gd, and Dy. Despite having the eight equations needed, geochemically reasonable weathering rates (e.g., positive primary mineral rates that reflect destruction) could not be achieved regardless of the two trace elements used in the mass balance calculations. For Brubaker Run, this is primarily attributable to the natural heterogeneity of the trace element concentrations within the host mineral grains, with trace element stoichiometries in some minerals varying by as much as an order of magnitude. Because the trace elements are hosted by a relatively small number of minerals, the computed weathering rates of other minerals become very sensitive to small variations in trace cation stoichiometry.

REE-rich epidote, garnet, and ankerite within the Brubaker Run watershed together host nearly all of the Ca in the bedrock, and completely dissolve at or near the weathering front. Consequently, approximately all of the Ca in bedrock is lost from the regolith. In bedrock the mole-percentages of Ca hosted by REE-rich epidote, garnet, and ankerite are 49 mol %, 4 mol %, and 43 mol %, respectively, and are determined by the modal abundance of the mineral in the bedrock and its Ca stoichiometry. The weathering rates of REE-rich epidote, garnet, and ankerite can be determined by distributing to each mineral that fraction of the total watershed Ca flux loss for which it is responsible based on its mole-percent Ca in bedrock. By using a basecation that is completely lost from the regolith, and knowing the mole-percentage of that element in the mineral(s) undergoing weathering, additional equations may be added to the mass balance matrix. We term this technique the “flux distribution method.” The flux distribution method eliminates the need for additional equations established using trace elements.

Based on the mineral weathering rates for the Brubaker Run watershed determined using the flux distribution method, the rates at which the weathering front penetrated the bedrock (the “saprolitization” rate) are 4.5 m My-1 and 6.5 m My-1 for chlorite and muscovite, respectively. These measured long-term average saprolitization rates compare very favorably with published theoretical values for the nearby northern Maryland Piedmont which range from 2.2 to 5.3 m

Geophysical Survey of an Abandoned Landfill

A group of students running a ground-penetratin radar survey

Landfills, in particular abandoned landfills which predate the Environmental Prorection Agency landfill regulations, are sources of groundwater contamination. Because they are non-invasive and are cost-effective, geophysical survey techniques are the preferred techniques for investigating such abandoned landfill. Geophysics is a branch of geology which uses physical measurements made at the Earth’s surface to gather information about the earth materials at depth. In the work reported here, students ran an integrated geophysical survey of a portion of a reclaimed landfill to study its internal structure. The landfill is located in a park in the city of Lancaster (Pennsylvania). Four different surveys were used:

  • an electrical conductivity survey
  • an electrical resistivity survey
  • a ground-penetrating survey
  • a magnetic field survey
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A student measuring the total magnetic field using a proton-precession magnetometer

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A group of studentsrunning a ground-penetratin radar survey

Results of the Survey:

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Magnetic anomaly map (nT) Electrical conductivity map (mS/m)

Interpretation:

  • Areas of high magnetic values which correlate with areas of high conductivity values are probably due to magnetic metallic object at depth.
  • Area with high conductivity but low magnetic values in the south-east corner is an indication of high moisture or high clay content of the ground.
  • The high magnetic value area on the east/northeast which has a low conductivity value may indicate the presence of construction debris from the city of Lancaster. The concrete with rebar is magnetic but not conductive.
  • The depth to the bottom of the landfill calculated from the depth electric resistivity profile is about 6 m at the site surveyed.
  • Due to penetration limitation, the ground-penetrating survey did no yield any useful information.

Geological Influences on Water Chemistry of Lititz Run

The Lititz Run Watershed, located in northern Lancaster County, Pennsylvania drains 11,133 acres of agricultural, urban, suburban, and natural land settings into Lititz Run (Landstudies, 1999). Ultimately, this water drains via the Conestoga and Susquehanna Rivers into the Chesapeake Bay. During the last several years, significant efforts have been made to restore this stream by stabilizing stream banks, installing fencing along some parts of the stream, and planting trees in the Riparian Zone.

This project assesses the role of the underlying rock units on stream water chemistry. Currently this project is characterizing nitrate contamination in the watershed. Preliminary studies indicate that geology plays the most significant role in the level of nitrate contamination. The Lititz Spring, which flows from the Conestoga Limestone, has nitrate levels 2.5 times the Environmental Protection Agency’s maximum contaminant level. However, springs from the siliciclastic units have nitrate levels well below the EPA level.

Last year, Tim Dempsey an engineering geology major, completed a departmental honors thesis on background nitrate levels in Lititz Run. In the picture to the left, Tim is analyzing water samples in the lab at Millersville. This year Joel Rogers, is building on Tim’s work by testing water quality of springs throughout the watershed. This information will be used to assess water quality in the watershed and to pinpoint areas where improvement can be made. We also hope that this work will help to improve restoration efforts not only in the Lititz Run Watershed but also in other carbonate and siliciclastic watersheds.

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