EnviroHorse

Rocking Trails and Soil Chemistry
By Adda Quinn

The following document has been scanned directly out of the original reference: Edgewood Park and Natural Preserve Master Plan Adopted May 1997, Environmental Services Agency Parks and Recreation Division, San Mateo County California. Appendix C-1 "Potential Impacts of Using Identified Non-Native Rocking Materials in Serpentine Grassland". It may contain minor errors as a result of the scanning process and a few clarification words have been added. The issue it addressed was an allegation made by members of the California Native Plant Society that rocking trails changed soil chemistry, and thus could affect plant growth. They presented NO SCIENCE to support this "fear", but the County ceased rocking trails because of it for over three years, anyway. In researching this paper, we spoke to over 18 scientists in the field, obtained references, and took all of our information to a Certified Professional Soil Scientist to evaluate and send a letter to the County during the Master Planning process for the park. Since 1997, trails are being rocked again in this property making year-round accessibility to this park possible for horses, primarily because of this information. This article proves that IF there is any change in soil chemistry due to trail rocking, it would only occur after hundreds of thousands of years. Reference documents cited can be obtained through EnviroHorse. Scanned text follows:

Dr. Ishwar P. Murarka
10446 San Femando Avenue
Cupertino, California 95014
408-996-9040 phone
408-996-1369 fax

CREDENTIALS

1979 M.B.A. The University of Chicago, Illinois
Management Science
1971 Ph.D. Oregon State University, Corvallis
Soil Science / Statistics
1968 M.S. Oregon State University, Corvallis
Soil Science / Statistics
1964 M.A. University of Calcutta, India
Geography
1962 B.A. University of Calcutta, India Honors in Geography
22 years of research with prestigious national & private research institutions
Life Membership in honorary academic society Sigma Xl chapter and at Large Membership in professional societies including:
Biometrics Society
American Society of Agronomy
Soil Science Society of America
American Society for Testing of Materials (D19 & C)34)
American Geophysical Union
Council for Agricultural Science and Technology - Life Member
Over 100 publications (bibliography available on request)

OBJECTIVE

As a Certified Professional Soil Scientist, I have been asked to review and comment on information provided for the purpose of determining the potential impact of using either basalt or granite quarry stone for trail construction and maintenance in a serpentine grassland. I have relied upon the soil descriptions provided by published literature and generally known to be associated with the rock types under consideration. I have neither taken soil samples, nor have I analyzed them I have been asked to prepare this paper for consideration by a non-technical audience. I am providing this information at no cost, as a public service.

COMMENTS

Weathering is defined as the mechanical disintegration and chemical break down of rock material that occurs when they are exposed to freeze-thaw cycles, pressure changes, exposure to water, chemical reactions, and other influences at the earth's surface. ' Both result in microscopic cracks along grain boundaries and within grains and occur at differing rates. Rates of rock weathering can be estimated by a variety of direct and indirect techniques. Direct techniques include laboratory studies. These tend not to be as accurate as other methods for predicting true environmental behavior, but are faster and less costly.2 The relationship between weathering, rind thickness and age also provides a direct measure of rock weathering rates. Indirect methods require solution of a mass balance equation to account for gains and losses of nutrients in a watershed, and are useful for all elements except phosphorus. They are usually long term field studies and are very costly. References reviewed involved a variety of methods.

Elemental release from rock weathering is highly variable and strongly dependent on the primary mineralogy of the rock. The larger the particle grain size, the more difficult the rock will be to weather. Small pieces of rock or small size minerals weather more rapidly than large pieces because of their greater surface area.3 In addition, degree of fracturing and chemical zonation of mineral grains are important controls.4

Because granite, basalt and serpentine are all base-saturated rocks (alkaline in nature with pH of about 8), their chemical composition is similar. Weathering processes should produce soils with similar elemental constituents. Both basalt and granite contain larger particle sized elements than does serpentine which will cause them to weather slower in comparison to the serpentine.5 Half inch diameter basalt or granite are commercially available from quarries in San Mateo County nearest the Edgewood Park and Natural Preserve which has the serpentinite grasslands of concern. Of these two quarry stones, the basalt mimics the chemistry of the serpentine soil closest, is the hardest of the three rocks being compared (due to higher iron content) and has the largest grain fractions. The weathering rate of granite has been calculated at 10-16 per meter squared per meter per second. This is extremely slow. Studies in California have demonstrated mineral grain depletion and destruction of pyroxene and hornblende take more than a hundred thousand (105) years. Depletion of these minerals takes several hundred thousand years in the same soils.6

To construct a simple analogy, if one took either of these quarry stones and placed them in a glass of water to stimulate decomposition, you might begin to see mechanical changes in 300-400 years. Chemical changes would take a thousand years. The geochemical and geological effect of use of either stone to rock trails in a serpentine soil where grasslands predominate will be non-detectable and probably not measurable except in geologic time scales.

In a practical sense, the use of these quarry stones in the trail bed of a park which will receive "light" use (defined as pedestrian, equestrian and bicycle) the effects of walking or riding on such rocks may create some minor forms of mechanical weathering. Over time, the most likely outcome is compaction of the rocks into the trail bed. This could have a positive effect of hardening the trail bed.

The climatic zone in which this property is located is Mediterranean, receiving 10-30 inches of rainfall annually, predominantly during winter months. It lies in the rain shadow of the Santa Cruz Mountains. These factors may further limit potential weathering of quarry stone used.

It is understood that trails account for 7.5 miles of track approximately three feet wide and that the majority of trails are not to be rocked. Less than 0.3% of the area of Edgewood is in trails in the grasslands of concern. The straying of on occasional stone from a trail bed into the grassland could conceivably alter the surface tension of clays and increase water infiltration potential. However, the effect would likely be much less than that caused by deer hooves during the course of their grazing.

Because transportation costs are often determined by weight, using close sources will be most cost - effective when use is indicated. Langley Quarry that supplies the basalt is located in Woodside; Pilarcitos Quarry that supplies the granite is in Half Moon Bay, both reasonably approximate to the site.

I believe that the information above is an accurate interpretation of the references provided. I understand that the original source material is being provided to the audience of interest along with this review and analysis.

(signed in original text)
Ishwar P. Murarka, Ph.D.
Certified Professional Soil Scientist
Date 7-30-96

REFERENCES PROVIDED
The following were provided as references:

Complete Soil Evaluation of Granite from Pilarcitos Quarry. 199. Soil and Plant Laboratory, Inc.

Charmichael, I., F. Turner and J. Verhoogen. 1974. Department of Geology and Geophysics UC Berkeley by McGraw-Hill, p.9 and p. 11.

Pertinent sections from Torrey and Torrey (initial master plan, 1982), Reid (EIA for golf course, 1993), Friends Draft (proposed master plan, 1996) and other comments (Lisa Hokholt, USDA SF Urban Team Nat. Res. Cons. Svc. of June 28, 1996).

Clayton, J., Nutrient Supply to Soil by Rock Weathering. Proceedings on the Impact of Intensive Harvesting on Forest Nutrient Cycling. USDA Forest Service, pp. 75-96.

Cleaves, E., D. Fisher, and O. Bricker. 1974. Chemical Weathering of Serpentinite in the Eastern Piedmont of Maryland. Geological Society of America Bulletin, Vol. 85, pp 437-444.

Colman, S. 1982. Chemical Weathering of Basalts and Andesites: Evidence from Weathering Rinds. USGS. United States Government Printing Office, Washington DC pp. 1-39 plus Appendices.

Crook, Jr. R. and A. Gillespie. 1986 Weathering Rates in Granitic Boulders Measured by P-Wave Speeds. Rates of Chemical Weathering of Rocks and Minerals. Academic Press, Inc. pp. 395-417.

Suarez, D. and J. Wood. Short and Long Term Weathering Rates of a Feldspar Fraction Isolated from an Arid Zone Soil. USDA-ARS Salinity Laboratory, Riverside California pending publication in Chemical Geology, pp. 1-24.

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