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Characteristics of Water-Well Yields in the Blue Ridge of Loudoun
County, Virginia

by David M. Sutphin[^1], Lawrence J. Drew^1^, John H. Schuenemeyer[^2],

and William C. Burton[^3]

Open-File Report 00-280

2000

This report is preliminary and has not been reviewed for conformity with
U.S. Geological Survey editorial standards (or with the North American
Stratigraphic Code). Any use of trade, product, or firm names is for
descriptive purposes only and does not imply endorsement by the U.S.
Government.

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Contents

Abstract\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\....3

Introduction\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....3

Procedure\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...5

Analysis\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...6

Analysis of
Variance\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...7

Variography\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....9

Conclusion\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....10

References
cited\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...12

Tables

Table 1. Rock types and their water wells in the Blue Ridge Province of
Loudoun County, Virginia, sorted by geologic age and
alphabetically\.....13

2\. Analysis of
variance\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....14

3\. Bonferroni adjustment for significance level between pairs of yields
with rock
types\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\....15

Figures

Figure 1. Locations of water wells in the Blue Ridge of Loudoun County,
Virginia\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....16

2\. Graph showing mean yields of wells in rock types in the Blue Ridge
of Loudoun County, Virginia, having 30 or more nonzero-yield
wells\.....17

3\. Variogram surface for 1,232 low-yield wells of the Marshall
metagranite (Ymb, Ymc, and Yml) in the Blue Ridge of Loudoun County,
Virginia\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....18

Abstract

Mean yields and the numbers of wells drilled are presented for 35 rock
types containing 4,115 wells in the Blue Ridge of Loudoun County,
Virginia. Analysis of variance of the well yields shows that Harper\'s
Formation phyllite and Catoctin Formation metabasalt have yields
significantly different from those of other rock types. Preliminary
variography has identified an anisotropy that roughly parallels the
general compressional trend of the Blue Ridge.

Introduction

Loudoun County, Virginia, is underlain by rocks of the Blue Ridge and
Piedmont Provinces and the Early Mesozoic Culpeper basin and
consequently has a diversity of rock types (Southworth and others,
1999). The rocks of the Blue Ridge extend from the western border of the
county to the east side of Catoctin Mountain and along the length of the
western half of the county from north to south. These rocks have been
divided into numerous geologic units (rock types) that have been
repeatedly penetrated by wells in the search for water for houses and
communities. In doing so, a great deal of well data have been generated.

In March, 2000, we were given by the State of Virginia the data set
containing information on the water wells in Loudoun County, Virginia,
and asked to determine whether rock type was a significant factor in the
variability seen in yields of the wells. Our research showed that the
mean yields of wells in some few rock types were significantly different
than the mean yields of wells in other rock types. We used data from
3,901 wells that had yields greater than zero, because we could not
distinguish the zero-yield wells from those where no yield data was
provided. The mean yields calculated for the rock types were higher than
the true mean yields, because we had necessarily left out the zero-yield
wells.

The purpose of this study is to use the water-well data to determine if
the rock types and geologic structures present in the Blue Ridge have
any bearing on the yields of the wells drilled into them, and to present
preliminary variography to determine if additional spatial analysis is
warranted.

The drilling of water wells is a complex socio-economic process having
numerous variables. Factors such as initial well yield, depth,
elevation, and rock type tell only part of the story. Other pertinent
factors include the drilling technology employed, the thickness of
overburden or regolith reported as depth to bedrock, and the topographic
location of the well site.

Over the last decade or so, drilling technology has changed with the
introduction of rotary drilling and hydrofracking. Rotary drills are
faster and can drill deeper than the cable tools that had been used
previously. Hydrofracturing or hydrofracking is the process of pumping
high-pressure water into a bedrock formation via the well to increase
the size and extent of existing bedrock fractures, thereby enlarging the
network of water-bearing fractures and increasing the area supplying
water to the well (NHDES, 2000, http://www.des.state.nh.us/ws-1-3.htm).

Daniel (1989) showed that wells drilled in draws and valleys have
average yields three times those of wells drilled on hills and ridges.
He found that wells in the most productive rock types have average
yields twice those of wells in the least productive types. Wells sited
in draws and valleys in the most productive rock types average five
times more yield than wells on hills and ridges in the least productive
types. Daniel (1989) demonstrated also that well diameter can have a
significant influence on yield and for a given depth, yield is directly
proportional to well diameter.

Along with this variability on yield imposed by topography, rock type
may exert an influence on whether a well provides the amount of yield
sought. With modern drilling technology, the required yield is usually
attained; however, a favorable rock type may make the expenditure of
additional funds to employ that technology unnecessary.

Procedure

The data set for the water wells of Loudoun County, Virginia, was
provided by the Virginia state geologist. Of the 23,245 wells in the
data set, 16,756 originate in rock units found in the Blue Ridge.
Unfortunately, for 12,827 of these wells, the reported yield is zero. A
number of zero-yield wells are to be expected in almost any area.
Experience in the Pinardville, NH, quadrangle (Drew and others, 2000),
for example, has shown that zero-yield wells may make up about 10 per
cent of the data. A closer look at the zero-yield well data reveals that
most of them have no reported secondary or tertiary yields, and other
data elements, such as elevation and casing depth, are missing. Because
it was felt that the vast majority of the zero-yield wells were in fact
wells with no reported yield data rather than dry holes, the zero-yield
wells were removed from the data set. This was done to remove the
ambiguity of not knowing whether a zero yield meant that there was no
water in the well or whether no yield data had been reported. However,
214 wells of the zero-yield wells have a prefix of \"WWDH\" in the
site-id field designating them as dry holes, so these wells were
included in the analysis.

After eliminating the several wells that had been hydrofracked, and one
additional well because its reported depth had been entered erroneously,
this analysis is based on data for 4,115 wells in the Blue Ridge of
Loudoun County, Virginia, with the 214 zero-yield wells making up about
5.2 per cent of the wells.

Using Geographical Information System (GIS) software, each well in the
data set was matched with its corresponding uppermost bedrock unit on
the geologic map of Loudoun County (Southworth and others, 1999),
attributes of the rock unit were added to the data set, and areas of the
rock units in the Blue Ridge were calculated. Many of the rock
attributes and much of the well data were unnecessary for this study and
were selectively removed. The resultant data set contained the following
11 fields from the original data set: site id, month, day, and year of
completion, base elevation, total yield, bedrock depth, total depth,
rock unit, UTM easting, and UTM northing.

Analysis

Thirty-five different rock units in the Blue Ridge of Loudoun County
contain at least one water well. These rocks range in age from the
Middle Proterozoic to the Jurassic and from metasediments such as
marble, metagreywacke, and meta-arkose to igneous rocks such as diabase
and granite, plus other metamorphic rocks such as metagranite and
phyllite. Descriptions of these units are found in Southworth and others
(1999) and are summarized in table 1.

Middle Proterozoic rocks in the Blue Ridge of Loudoun County have a
total area of about 487.8 km^2^, with the three units of Marshall
metagranite, (Ymb, Ymc, and Yml) making up 194.3 km^2^ of that. Late
Proterozoic rocks account for 282.9 km^2^ with the five units of
Catoctin Formation (Zc, Zcm, Zcp, Zcr, and Zcs) accounting for 201.9
km^2^ of that area. Cambrian rocks make up 51.8 km^2^, and Jurassic age
rocks make up 3.0 km^2^ of the Blue Ridge of Loudoun County. Total area
of the Blue Ridge is 825.5 km^2^ or about 62 per cent of the county.

The locations of the wells in the Blue Ridge of Loudoun County are shown
in figure 1. In the figure, it is obvious that the wells are not
distributed evenly throughout the Blue Ridge. They form clusters in
communities and subdivisions, strings of wells outline highways, and
some rock units have many hundreds of wells while others have as few as
one well.

Wells in Middle Proterozoic age rocks total 2,717 and make up more than
66 per cent of the wells in the data set. In this age class, the
Biotitic Marshall Metagranite (Ymb) has the most wells drilled (868),
about 21 percent of the total. Wells in Late Proterozoic age rocks total
1,171 and make up more than 28 per cent of the number of nonzero wells.
The Late Proterozoic age rock unit having the most wells is Catoctin
Formation metabasalt (Zc) with 724 wells. There are 211 wells in
Cambrian rocks or 5 per cent of the total number of zero-yield wells,
with Harpers Formation phyllite (Ch) rocks having the majority of the
wells. The 16 wells in Jurassic diabase dikes (Jd) account for less than
0.4 per cent of the nonzero-yield wells.

The mean yield of the nonzero-yield wells for each rock type present in
the Blue Ridge of the county are shown also in table 1. Total mean yield
for all rock types is 15.52 gpm, but the mean yield of an individual
rock type may vary from 30.00 gpm for the one well drilled into Catoctin
metasandstone (Zcs) and 24.82 gpm for the 57 wells drilled into Swift
Run metagraywacke (Zss) to 3.50 gpm for the 6 wells drilled into
Tomstown Formation dolostone (Ct). Note that the 868 wells drilled into
Biotitic Marshal Metagranite (Ymb) and the 603 wells drilled into
Garnetiferous metagranite(Ygt) average 16.72 and 16.345 gpm,
respectively, while the 724 wells drilled into Catoctin Formation
metabasalt (Zc) have a much lower yield of 11.72 gpm.

Analysis of Variance

A one-way analysis of variance was used to test the hypothesis that the
mean well yields are not significantly different across rock types. This
is a standard procedure to test for differences between means. It
partitions the total variability of yield into variability between rock
types and within rock types. This latter term is often called the error.
The null hypothesis of no significant difference in the mean yields was
tested for the 16 rock types in which 30 or more wells had been drilled.
A minimum sample size of 30 was chosen to ensure the stability of the
results. The results of the hypothesis test are shown in table 2 where
it can be seen that the null hypothesis was rejected at a probability
level of less that one chance in a thousand of being true. That is,
there is a least one of the observed comparisons that can be made
between the observed mean yields across the 16 rock types that is
sufficiently large as to have occurred at random less than one time in a
thousand. When the null hypothesis of equal means is true, the mean
square for rock type (table 2) is to within sampling variability equal
to the mean square error. The ratio of these two quantities, known as
the F-ratio, becomes larger when the null is more likely to be false, as
it is for this data set.

Inspection of the Bonferroni[^4] probabilities for each of the pair
comparisons (table 3) shows that there are 11 such pairs of mean
differences that contributed to the rejection of the null hypothesis
that there is no difference among the mean yields. The most striking
comparisons are between Harper\'s Formation phyllite (Ch) and the Middle
and Late Proterozoic rocks, where we find nine possible comparisons
among the mean yields to be statistically significant, that is, having a
value of 0.050 or less. The second strongest and only other set of
differences is between Catoctin Formation metabasalt (Zc) rock types and
the other rock types, where four significant comparisons exist (with
Ygt, Ymc, and Ch).

The significance between the means is shown graphically in Figure 2,
where it can be seen that the mean yield and the confidence interval for
Harper\'s Formation phyllite (Ch) is out of the range of the confidence
limits for all but one other rock types\-- the Antietam quartzite
(Ca)\-- as was shown by the significant probabilities in column 2 of
table 3. To a similar degree, Swift Run metagraywacke (Zss) has a mean
yield that is higher than all other rock types but a mean yield that is
significantly higher than only Harper\'s Formation phyllite (Ch). To a
much lesser degree, Antietam quartzite (Ca) has a mean yield that is
less than all rock types other than Harper\'s Formation phyllite (Ch),
however, the wide range of the error bars precludes well yield from the
Antietam quartzite from being significantly different than any other
rock type. Coarse-grained Marshall metagranite (Ymc) has a mean yield
that is greater than most other rock types and narrow confidence limits
that overlap with few of those other rock types.

Variography

In water-well variography, only data from low-yield wells (wells with
yields of less than 40 gpm) are used, because their variogram surfaces
have been shown by Drew and others (1999) in the Pinardville, NH,
quadrangle to have anisotropies that imply a spatial continuity between
the yield of one low-yield well to that of another. Thus we have chosen
to perform the analysis using the same methods for the Blue Ridge of
Loudoun County. These anisotropies are linked to the local bedrock
geology. The variogram surface of the high-yield wells, those yielding
40 gpm or higher, do not have such anisotropies, because there is no
spatial continuity between the yield of one high-yield well to that of
another.

The software used in variography (Variowin) has limits to the number of
pairs that can be compared, so subsets of the well data were used in the
variography.

Preliminary variography with the above subset confirms links between the
bedrock geology and the water-well yields similar to those first
described in Drew and others (1999) for the Pinardville, NH, quadrangle.
Figure 3 shows the variogram surface for the 1,232 nonzero low-yield
wells in rocks of the Marshall granite (Ymb, Ymc, and Yml). There is an
anisotropy trending about N54E through the middle of the variogram,
which may correspond to the general compressional trend of the Blue
Ridge. The anisotropy is not clearly defined, perhaps because the
surface is a composite of data from three rock types. These rock types,
being of different textures, may have somewhat different geologic
histories and fabrics that influence the connectivity of the wells. Weak
patches of local lows trend at right angles to the overall trend of the
anisotropy, which may be indicative of another tectonic fabric affecting
well interconnectivity, such as the locally northwest-trending contacts
between the Middle Proterozoic units. Further variography with other
subsets, broken out by rock type and possibly adjusted for trends in
yields brought about by improved drilling technology, may be able to
reduce the anisotropy into its components.

Conclusion

It was been shown statistically that there are significant differences
in water-well yields across rock types in the Blue Ridge of Loudoun
County, Virginia. Wells drilled in Harper\'s Formation phyllite, for
example, have significantly lower yields than wells in any other rock
type, and yields from wells in the Catoctin formation metabasalt are
significantly different than yields from wells in rocks of two rock
types other than Harper\'s Formation phyllite. The greatest range in
yields is found in the Cambrian to Late Proterozoic metasedimentary and
meta-igneous cover-sequence rocks of the Blue Ridge, whereas the mostly
granitic Middle Proterozoic rocks exhibit a much narrower range of
yields. This demonstrates that bedrock lithology does influence yield in
western Loudoun County.

One composite variogram surface for the Marshall granite has shown that
yields of low-yield wells have anisotropies linked to the bedrock
geology of the Blue Ridge of Loudoun County. Further variography can be
done to disaggregate the data by rock type or geographic area to further
refine the variograms so that additional anisotropies can be recognized.
Rock fracture patterns and fabrics seen in the field may contribute to
these anisotropies or the anisotropies may lead to identification of
additional patterns in the rocks.

References Cited

Daniel, C.C., III, 1989, Statistical analysis relating well yield to
construction practices and siting of wells in the Piedmont and Blue
Ridge Provinces of North Carolina: U.S. Geological Survey Water-Supply
Paper, 2341---A, 27 p.

Drew, L.J., Karlinger, M.R., Armstrong, T.R., and Moore, R.B., 1999,
Relations between igneous and metamorphic rock fracture patterns and
groundwater yield from the variography of water-well
yields---Pinardville quadrangle, New Hampshire: Natural Resources
Research, v. 8, no. 2, p. 137-152.

Southworth, Scott, Burton, W.C., Schindler, J.S., and Froelich, A.J.,
1999, Digital geologic map of Loudoun County, Virginia: U.S. Geological
Survey Open-File Report, 99-150, scale 1:50,000.

[^1]: U.S. Geological Survey, 954 National Center, Reston, VA 20192

[^2]: U.S. Geological Survey, c/o Department of Geography, University of
    Delaware, Newark, DE 19716

[^3]: U.S. Geological Survey, 926 National Center, Reston, VA 20192

[^4]: Bonferroni is a statistical procedure designed to protect against
    incorrectly rejecting the null hypothesis.