U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5043

Prepared in cooperation with the

Maryland Department of the Environment

Statistical Classification of Hydrogeologic Regions 

in the Fractured Rock Area of Maryland and Parts 

of the District of Columbia, Virginia, West Virginia, 

Pennsylvania, and Delaware
Cover. Location of study area for the statistical classification of hydrogeologic regions.
Statistical Classification of Hydrogeologic 

Regions in the Fractured Rock Area 

of Maryland and Parts of the District 

of Columbia, Virginia, West Virginia, 

Pennsylvania, and Delaware

By Brandon J. Fleming, Andrew E. LaMotte, and Andrew J. Sekellick 

Prepared in cooperation with the

Maryland Department of the Environment

Scientifc Investigations Report 2013–5043

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Fleming, B.J., LaMotte, A.E., and Sekellick, A.J., 2013, Statistical classification of hydrogeologic regions in the 

fractured rock area of Maryland and parts of the District of Columbia, Virginia, West Virginia, Pennsylvania, and 

Delaware: U.S. Geological Survey Scientific Investigations Report 2013–5043, 14 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Previous Investigations........................................................................................................................2

Study Methods ...............................................................................................................................................3

Geographic Data...................................................................................................................................3

Statistical Analysis Methods ..............................................................................................................4

Statistical Classification of Hydrogeologic Regions................................................................................5

Principal Components Analysis..........................................................................................................5

Cluster Analysis.....................................................................................................................................6

Details of the New Classification of the Crystalline Rock Area....................................................8

Summary........................................................................................................................................................12

Acknowledgments.......................................................................................................................................12

References Cited..........................................................................................................................................12

Appendix........................................................................................................................................................14
iv

Figures

1. Map showing location of study area for the statistical classification of 

hydrogeologic regions .................................................................................................................2

2. Map showing location of catchment study units derived from NHDPlus...........................4

3. Scree plot showing the Eigenvalues of the principal components......................................5

4. Dendrogram showing the hierarchical grouping of NHDPlus catchments 

by hydrogeologic region..............................................................................................................6

5. Boxplots showing the distribution of input variables for each hydrogeologic 

region..............................................................................................................................................7

6. Map showing hydrogeologic regions derived from hierarchical clustering 

of NHDPlus catchments by principal components for entire study area............................9

7. Map showing hydrogeologic regions derived from hierarchical clustering 

of NHDPlus catchments by principal components for Maryland.......................................10

8. Boxplots showing (A) recharge, (B) Precipitation minus Potential 

Evapotranspiration, (C) base-flow index (BFI), and (D) sand for Piedmont 

Crystalline Uplands and Blue Ridge and Piedmont Crystalline Lowlands 

hydrogeologic regions ...............................................................................................................11

Tables

1. Variance explained by principal components .........................................................................5

2. Principal component loadings....................................................................................................5

3. Description of hydrogeologic regions.......................................................................................8

4. Statistically significant differences of descriptive variables between the 

two crystalline-rock hydrogeologic regions ..........................................................................12
v

Conversion Factors and Datums

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F = (1.8 x °C) + 32

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88). 

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Water year in USGS reports is the 12-month period October 1 through September 30. The water 

year is designated by the calendar year in which it ends and includes 9 of the 12 months. Thus, 

the year ending September 30, 2002, is the “2002 water year.”

Statistical Classification of Hydrogeologic Regions 

in the Fractured Rock Area of Maryland and Parts 

of the District of Columbia, Virginia, West Virginia, 

Pennsylvania, and Delaware

By Brandon J. Fleming, Andrew E. LaMotte, and Andrew J. Sekellick 

Abstract

Hydrogeologic regions in the fractured rock area of 

Maryland were classified using geographic information 

system tools with principal components and cluster analyses. 

A study area consisting of the 8-digit Hydrologic Unit 

Code (HUC) watersheds with rivers that flow through the 

fractured rock area of Maryland and bounded by the Fall 

Line was further subdivided into 21,431 catchments from the 

National Hydrography Dataset Plus. The catchments were 

then used as a common hydrologic unit to compile relevant 

climatic, topographic, and geologic variables. A principal 

components analysis was performed on 10 input variables, 

and 4 principal components that accounted for 83 percent of 

the variability in the original data were identified. A subsequent cluster analysis grouped the catchments based on four 

principal component scores into six hydrogeologic regions. 

Two crystalline rock hydrogeologic regions, including large 

parts of the Washington, D.C. and Baltimore metropolitan 

regions that represent over 50 percent of the fractured rock 

area of Maryland, are distinguished by differences in recharge, 

Precipitation minus Potential Evapotranspiration, sand content 

in soils, and groundwater contributions to streams. This classification system will provide a georeferenced digital hydrogeologic framework for future investigations of groundwater 

availability in the fractured rock area of Maryland. 

Introduction

The U.S. Geological Survey (USGS), in cooperation with 

the Maryland Department of the Environment Water Supply 

Program (MDE WSP), began the Maryland Fractured Rock 

Water Supply Study in 2009. As the first phase of this study, 

an initial framework describing regions with similar hydrogeologic characteristics was developed. The methods and results 

used to develop the framework are described in this report. 

In Maryland, the hydrogeologic systems can be most 

generally divided into physiographic provinces defined by 

Fenneman and Johnson (1946), with the Fall Line separating 

the unconsolidated layered system of aquifers and confining 

units in the Coastal Plain from the spatially varying igneous 

and metamorphic bedrock of the Piedmont, essentially dividing the State in half. West of the Piedmont lie the Blue Ridge, 

Valley and Ridge, and Appalachian Plateau Physiographic 

Provinces that consist of fractured siliciclastic, igneous and 

metamorphic, and carbonate formations. Groundwater in 

these rocks moves chiefly through networks of fractures. The 

water-bearing materials in this area are generally anisotropic 

and heterogeneous, which has made assessing groundwater 

availability difficult. The area inclusive of these provinces 

west of the Fall Line will hereafter be referred to as the “fractured rock area of Maryland” (fig. 1). A report by the Advisory 

Committee on the Management and Protection of the State’s 

Water Resources (Wolman, 2008) raised awareness of potential water supply issues in the fractured rock area of Maryland. 

Issues of particular concern to the MDE WSP are increased 

groundwater demand from expected population growth near 

Baltimore, Maryland and Washington, D.C.; the impacts of climate variability on groundwater supplies; and adverse impacts 

on the aquatic health of streams from water withdrawals. 

The fractured rock area of Maryland has significant variations in factors affecting groundwater availability including 

climate, topography, and geology. As such, groundwater and 

surface-water availability west of the Fall Line is difficult to 

determine at a regional scale. To better manage the groundwater resources in the fractured rock area of Maryland, a 

common hydrogeologic framework for resource managers 

and researchers is needed to integrate information on factors 

relating to groundwater availability, hydrologic processes, 

aquatic health, and response to climate variability. In addition, 

the framework needs to be in digital format and georeferenced 

for use in the Maryland Fractured Rock Aquifer Information 

System (FRAIS), a geographic information system (GIS) 

designed to support the water supply study in the region 

underlain by fractured rock in Maryland (Fleming and others, 

2012). The hydrogeologic framework can then be used as a 

starting point for subsequent research efforts to quantify water 

availability in the fractured rock area of Maryland. 
2 Statistical Classification of Hydrogeologic Regions in the Fractured Rock Area of Maryland and Parts of Adjacent States

Figure 1. Location of study area for the statistical classification of hydrogeologic regions.

Purpose and Scope

This report describes the results of a statistical analysis 

designed to identify regions with similar hydrogeologic 

characteristics in the fractured rock areas of Maryland and 

adjacent states. The study area is bounded by the Fall Line to 

the east and the outer boundaries of the 8-digit Hydrologic 

Unit Code (HUC) watersheds, which contribute streamflow 

through the fractured rock area of Maryland (fig.1). The study 

area was selected based on naturally occurring hydrologic 

and physiographic boundaries rather than political boundaries and includes parts of the District of Columbia, Virginia, 

West Virginia, Pennsylvania, and Delaware with similar 

physical environments to areas in Maryland. This classification will provide a hydrogeologic framework in support of the 

Maryland Fractured Rock Water Supply Study. 

Previous Investigations

Studies combining areas of similar geologic, hydrologic, 

or hydrogeochemical characteristics have been conducted at 

multiple spatial scales. Non-statistical methods for grouping 

physiographic provinces in the United States are well known 

(Fenneman and Johnson, 1946). For the Chesapeake Bay 

watershed, hydrogeomorphic regions were developed on 

the basis of rock type and physiography to look at baseflow nitrate loads of nontidal streams (Bachman and others, 

1998; Brakebill and Kelley, 2000). Hydrogeologic units were 

categorized for Allegany and Washington Counties (Slaughter 

and Darling, 1962), and a recent physiographic map of 

Maryland based on detailed local knowledge of the geology 

shows both the Coastal Plain and fractured rock areas (Reger 

and Cleaves, 2008). 
Study Methods 3

Statistical methods for grouping terrains that have been 

implemented more recently have taken advantage of modern 

computing capabilities. GIS and multivariate statistical 

analysis methods were used to develop hydrologic landscape 

regions for the United States (Wolock and others, 2004). The 

authors performed a principal components analysis (PCA) 

and cluster analysis to delineate 20 distinct regions based on 

land-surface form, geologic texture, and climate. The hydrologic landscape regions were compared to other regional 

frameworks and found to be as good (if not better) in delineating regions of distinct land-surface form and geologic texture. 

PCA and cluster analysis were used by Armstrong and others 

(2008) to identify basins with similar hydrologic characteristics based on ecologically relevant streamflow indices for 

least altered sites in southern New England. In central Idaho, 

Lipscomb (1999) used basin characteristics as input variables 

for a PCA as the basis for a data-collection network for quantifying in-stream flows. In Maryland, Preston (2000) applied 

cluster analysis to identify hydrochemical response units. 

These units were later used by the State to design a waterquality management plan. 

Study Methods

GIS methods were used to compile relevant data for 

catchments derived from the National Hydrography Dataset 

Plus (NHDPlus) (Horizon Systems, 2010) in order to perform 

a PCA on hydrogeologic variables (fig. 2). Catchment study 

units, nested within the 8-digit HUCs and derived from the 

NHDPlus dataset are hydrologically defined units that provide 

enough resolution to describe known hydrogeologic variability 

for areas within Maryland and adjacent states. The catchment 

study units ranged in area from less than 1 to 76 square 

kilometers (km2) with a mean of 2.4 km2 and a median of 1.5 

km2. A clustering algorithm was then applied to the principal 

components scores for each of 21,431 catchment study units in 

the study area. 

Geographic Data 

The extent of the fractured rock study area was delineated 

by 8-digit HUC watersheds that contribute streamflow into 

the non-Coastal Plain part of Maryland (fig. 1). Catchment 

study units were defined by NHDPlus catchments (fig. 2). 

Characteristics such as topography, hydrology, and geology, 

among others, were assigned to each NHDPlus catchment 

by use of a GIS. Data layers included Precipitation minus 

Potential Evapotranspiration (PET), base-flow index (BFI), 

recharge, mean elevation, mean slope, relief, percent sand 

in soils, and geology (carbonate rock, siliciclastic rock, and 

crystalline rock, for a total of 10 input variables. Each input 

variable was converted to a 30-meter (m) grid to match the 

resolution of the elevation base-data layer and used as input 

for area statistics. Input variable values for each NHDPlus 

catchment were calculated in the GIS and exported to tables to 

be used as input for the PCA and cluster analysis. 

An 800-m, long-term (30-year annual average) precipitation grid produced by the PRISM Climate Group (Daly and 

others, 2008) was converted to a 30-m grid. A 1-kilometer 

(km) estimated PET grid developed from long-term (30-year) 

climate data by the USGS (Wolock and others, 2004) was 

converted to a 30-m grid and subtracted from the precipitation 

grid to produce Precipitation minus PET, which was treated as 

a single variable and summarized by catchment study unit. 

The hydrologic characteristics BFI and recharge were 

converted from 1-km grids to 30-m grids. Base flow is the 

component of streamflow that can be attributed to groundwater discharge into streams. The BFI is the ratio of base flow to 

total streamflow, expressed as a percentage. The source dataset 

was the base-flow index for the conterminous United States 

(Wolock, 2003a). Recharge represents the mean annual natural 

groundwater recharge, in millimeters. Recharge is derived 

from BFI (representing long-term natural groundwater discharge to streams) and long-term mean annual runoff values. 

The source dataset for recharge was estimated mean annual 

natural ground-water recharge in the conterminous United 

States (Wolock, 2003b). All hydrologic characteristics were 

summarized for each catchment study unit. 

The topographic features—mean elevation, mean slope, 

and relief—for each catchment study unit were derived from 

the USGS 30-m National Elevation Dataset (Gesch and others, 2009). Mean elevation values, in meters, were calculated 

from raster cells within each catchment study unit. Slope was 

defined as percent rise over run, for each raster cell, and the 

mean value calculated for each catchment study unit. Relief 

is the maximum elevation of a catchment study unit minus its 

minimum elevation, in meters.

Percent sand was derived from the U.S. Department 

of Agriculture’s STATSGO soils database (Wolock, 1997). 

Percent sand was used as a proxy of the soil’s permeability 

(the higher the percentage of sand, the more permeable the 

soil). Percent sand was calculated as a mean percentage by 

catchment study unit.

The geologic layer is a compilation of igneous and metamorphic crystalline rock, carbonate rock, and siliciclastic rock. 

This grid was made by overlaying a map layer of carbonate 

rocks compiled from State geologic maps (Dicken and others, 

2005; Nicholson and others, 2005) onto a map layer made up 

of igneous and metamorphic crystalline rocks and siliciclastic 

rocks modified from the USGS database of principal aquifers 

(U.S. Geological Survey, 2003). This resulted in three geology 

input variables: carbonate rock, siliciclastic rock, and crystalline rock. The dataset was selected because of its consistency 

across political boundaries. Each rock type was treated as a 

separate variable and calculated as a percentage by catchment 

study unit.
4 Statistical Classification of Hydrogeologic Regions in the Fractured Rock Area of Maryland and Parts of Adjacent States

Figure 2. Location of catchment study units derived from NHDPlus.

Statistical Analysis Methods

A PCA was performed to examine the relations among 

the 10 variables described above and reduce them to a statistically significant number of components (Hamilton, 1992; 

SAS Institute, 2002). The central idea of a PCA is to reduce 

the dimensionality of a dataset consisting of a large number 

of interrelated variables, while retaining as much as possible 

of the variation present in the dataset (Jolliffe, 2010). The 

variables with high loadings in a principal component are 

correlated among themselves and less correlated with other 

variables. For this study, two criteria were used to determine 

the number of principal components to use in the subsequent 

cluster analysis: first, the proportion of variance explained by a 

component’s Eigenvalue (variance of the original components) 

had to be above 10 percent (table 1), and second, a scree 

plot showing the break in slope of the Eigenvalues between 

components 1–4 and components 5–10 for the 10 principal 

components (fig. 3). Each input variable has a value for each 

principal component, called a principal component loading. 

Specific loadings of variables for the four principal components are listed in (table 2). 

The results of the PCA are used in a cluster analysis to 

aggregate the NHDPlus catchments into groups with similar 

component scores. The cluster analysis was performed using 

Ward’s minimum variance method, an agglomerative hierarchical clustering technique (McGarigal and others, 2000; 

Ward, 1963). Individual observations are considered unique 

clusters and are further grouped into larger clusters on the 

basis of the Euclidean distance between data values of the 

catchment at its centroid. A similar approach was used by 

Preston (2000) and Lipscomb (1999). The selection of the 

final number of clusters is dependent on the study goals, in 

this case, attempting to account for the most information in the 

data while maintaining a minimum number of groupings for 

easy interpretation. 
Statistical Classification of Hydrogeologic Regions 5

Table 1. Variance explained by principal components.

Principal 

component Eigenvalue

Proportion of 

variance 

explained

Cumulative 

variance 

explained

1 3.40 0.34 0.34

2 2.05 0.21 0.55

3 1.58 0.16 0.70

4 1.28 0.13 0.83

Figure 3. Scree plot showing the Eigenvalues of the principal 

components.

Statistical Classification of 

Hydrogeologic Regions 

The results of the PCA and subsequent cluster analysis 

are presented below. Six hydrogeologic regions are presented 

for the fractured rock area of Maryland. The characteristics of 

the hydrogeologic regions derived from the principal components and cluster analysis are described. Distinction is made 

between two hydrogeologic regions both underlain by crystalline rock, and their differences are explained. 

Principal Components Analysis

The results of the PCA are four principal component 

scores for each NHDPlus catchment. The four principal 

components are ranked by the variance they account for (table 

1), and the loadings of the input variables on each principal 

component are shown in table 2. Loadings in table 2 are 

rotated using the varimax rotation, which aids in interpreting 

the principal components (Jolliffe, 2010). The first four principal components account for 83 percent of the variability in 

the original data. Subsequent principal components contribute 

decreasing amounts of information (fig. 3), while adding complexity to the PCA results. Therefore, four components were 

selected for use in the cluster analysis. 

Table 2. Principal component loadings.

[--, loadings are less than 0.5; Precipitation-PET, Precipitation minus Potential Evapotranspiration]

Variable Principal component

1 2 3 4

Base-flow index -0.50 -- -- --

Crystalline rock -0.48 -- -- --

Siliciclastic rock 0.62 -- -- --

Precipitation-PET -- 0.63 -- --

Recharge -- 0.64 -- --

Sand -- -- 0.58 --

Carbonate rock -- -- -0.69 --

Elevation -- -- -- 0.36

Relief -- -- -- 0.63

Slope -- -- -- 0.59
6 Statistical Classification of Hydrogeologic Regions in the Fractured Rock Area of Maryland and Parts of Adjacent States

Cluster Analysis

The cluster analysis resulted in six clusters that represent 

hydrogeologic regions. The hierarchical grouping of the first 

100 clusters is shown in figure 4, with the hydrogeologic 

regions color coded, accounting for 65 percent of the information within the principal component data. Six hydrogeologic 

regions account for much of the variability in the principal 

component dataset while producing a set of hydrogeologic 

regions that is relatively easy to describe. The distribution of 

input variable values for the catchments in each hydrogeologic 

region is shown in figure 5.

The Piedmont Crystalline Uplands (PCU) hydrogeologic 

region covers 26 percent of the fractured rock area of 

Maryland (table 3). It is underlain by igneous and metamorphic crystalline rock and includes areas near the divide 

between the Monocacy and Patapsco watersheds, much of 

northern Baltimore, Harford, and Cecil Counties in Maryland 

and parts of southeastern Pennsylvania. Compared to the other 

regions, it has high recharge, and a high base-flow index, 

meaning much of the streamflow in these areas is derived from 

groundwater. 

The Blue Ridge and Piedmont Crystalline Lowlands 

(BRPCL) hydrogeologic region covers 28 percent of the fractured rock area of Maryland. It is also underlain by igneous 

and metamorphic crystalline rock, and includes much of the 

Blue Ridge Physiographic Province (fig. 6) (Fenneman and 

Johnson, 1946) including parts of western Frederick County, 

Maryland. Central Carroll and eastern Frederick Counties 

are in this region, as well as most of Montgomery County. 

Parts of Howard and Baltimore Counties nearest the Fall Line 

within the Coastal Plain are also in this hydrogeologic region. 

Although the underlying rock type is similar to the PCU, there 

are several differences, including base-flow index, and they 

will be discussed later in the report. 

Figure 4. Dendrogram showing the hierarchical grouping of NHDPlus catchments by hydrogeologic region.
Statistical Classification of Hydrogeologic Regions 7

Figure 5. Boxplots showing the distribution of input variables for each hydrogeologic region (n = number of 

catchment units in each hydrogeologic region).
8 Statistical Classification of Hydrogeologic Regions in the Fractured Rock Area of Maryland and Parts of Adjacent States

Table 3. Description of hydrogeologic regions.

[Precipitation-PET, Precipitation minus Potential Evapotranspiration]

Hydrogeologic region Abbreviation

Percent of 

Maryland 

fractured 

rock 

terrain

Description

Piedmont Crystalline Uplands PCU 26

Regions of crystalline rock with low elevation, low relief 

and low slope, and high base-flow index and high 

recharge.

Blue Ridge and Piedmont Crystalline Lowlands BRPCL 28 Regions of crystalline rock with low relief, low elevation, 

and low slope and sandy soils.

Siliciclastic Ridges SR 4 Regions of siliclastic rock with high relief and slope, 

relatively high elevations, and sandy soils.

Appalachian Plateau AP 17

Regions of siliclastic rock with the highest elevation and 

Precipitation-PET along with relatively high recharge 

and low base-flow index.

Carbonate Valleys CV 10

Regions of carbonate rock with the lowest percent sand, 

low slope, low relief, and low elevation and relatively 

high base-flow index.

Siliciclastic Valleys SV 16 Regions of siliclastic rock with low relief, low elevation, 

moderately sandy soils, and low Precipitation-PET.

The Siliciclastic Ridges (SR) hydrogeologic region covers only 4 percent of the fractured rock area of Maryland, and 

is by far the smallest in areal extent in the State. It is underlain 

by mostly siliciclastic rock with some carbonate rocks. In 

Maryland, catchments falling into this hydrogeologic region 

are in Washington, Allegany, and Garrett Counties. This region 

has high total elevation, steep slopes, and the greatest withincatchment relief. 

The Appalachian Plateau (AP) hydrogeologic region 

covers 17 percent of the fractured rock area of Maryland and 

includes western Allegany and most of Garrett Counties 

(fig. 7). This region is almost exclusively underlain by siliciclastic rock. It has the highest mean elevation, the highest 

Precipitation minus PET values, and high recharge. 

The Carbonate Valleys (CV) hydrogeologic region covers 

10 percent of the fractured rock area of Maryland. Although 

only 10 percent of the land area, this is an important region in 

central and western Maryland because the valleys underlie the 

cities of Hagerstown and Frederick. Sand content is lowest in 

this region and base-flow index is high, relative to the other 

hydrogeologic regions (fig. 5). 

The Siliciclastic Valleys (SV) hydrogeologic regions 

account for 16 percent of the fractured rock area of Maryland. 

Catchments in this hydrogeologic region are underlain by 

siliciclastic rocks and sand content in soil is moderately 

sandy. Precipitation minus PET, recharge, and base-flow index 

are relatively low, likely because of topographic shadow 

effect. These regions include the Gettysburg Basin, and part 

of northern Frederick and Carroll Counties, as well as the 

Culpepper Basin, with parts of southern Frederick and western 

Montgomery Counties. Parts of Washington and most of 

Allegany County are included in this hydrogeologic region. 

Details of the New Classification of the 

Crystalline Rock Area

Population growth and accompanying water demands are 

anticipated near the Baltimore and Washington, D.C. metropolitan areas, much of which are areas underlain by fractured 

crystalline rock. As described by Fleming and others (2012), it 

is important to understand the factors affecting reliable yields 

of individual groundwater supplies and to understand the 

potential impacts from newly developed groundwater supplies 

to existing water users. This report describes a hydrogeologic 

classification for all of Maryland and parts of adjacent states 

underlain by fractured rock, but also shows a new classification for the crystalline rock area which makes up over 50 

percent of the fractured rock area in Maryland (table 3). The 

differences between the two crystalline rock hydrogeologic 
Statistical Classification of Hydrogeologic Regions 9

Figure 6. Hydrogeologic regions derived from hierarchical clustering of NHDPlus catchments by principal components for entire 

study area. (Dashed lines are from Fenneman and Johnson, 1946).

regions PCU and BRPCL are shown in figure 8. PCU has a 

higher base-flow index, recharge, and Precipitation minus PET 

than BRPCL (table 4). PCU includes headwater catchments 

along the divide of the Monocacy River and Patapsco River 

watersheds in Maryland (fig. 7). Headwater areas can be more 

dependent on groundwater discharge to generate streamflow 

and maintain base flow (Winter, 2007). This new classification 

in the crystalline rock areas of Maryland identifies those areas 

where groundwater discharge to streams is a larger component 

of total streamflow and supports results from hydrograph 

separation analysis performed by the MDE WSP (Patrick 

Hammond, Maryland Department of the Environment, Water 

Supply Program, oral commun., 2011). The differences 

between PCU and BRPCL described in this report indicate 

that streamflow in PCU may be more sensitive to changes in 

groundwater levels because groundwater is a larger component 

of the water budget. 
10 Statistical Classification of Hydrogeologic Regions in the Fractured Rock Area of Maryland and Parts of Adjacent States

Figure 7. Hydrogeologic regions derived from hierarchical clustering of NHDPlus catchments by principal components for 

Maryland.
Statistical Classification of Hydrogeologic Regions 11

Figure 8. Boxplots showing (A) recharge, (B) Precipitation minus Potential Evapotranspiration, (C) base-flow index (BFI), and 

(D) sand for Piedmont Crystalline Uplands and Blue Ridge and Piedmont Crystalline Lowlands hydrogeologic regions.
12 Statistical Classification of Hydrogeologic Regions in the Fractured Rock Area of Maryland and Parts of Adjacent States

Table 4. Statistically significant differences of descriptive variables between the two crystalline-rock hydrogeologic regions.

[<, less than]

Variable Unit Median Piedmont 

Crystalline Uplands

Median Blue Ridge and 

Piedmont Crystalline 

Lowlands

p-values

Statistically significant 

differences 

(alpha = 0.0001)

Mean elevation Meters 143.485 141.900 0.1306 No

Total relief Meters 72.980 65.850 0.0072 No

Slope Percent rise/run 8.195 8.270 0.1059 No

Climate Millimeters 442.455 350.410 < 0.0001 Yes

Sand Percent 27.050 34.760 < 0.0001 Yes

Recharge Millimeters 259.845 187.370 < 0.0001 Yes

Base-flow index Percent 57.000 48.920 < 0.0001 Yes

Summary

A hydrogeologic framework for the fractured rock area of 

Maryland was derived using geographic information system 

applications and multivariate statistical methods. The goal 

of this new classification is to provide future investigations 

of groundwater resources with a hydrogeologic framework. 

Further investigation is needed within these hydrogeologic 

regions to determine how other factors, such as overburden 

thickness, topographic position, and well yields relate to one 

another. The study area is bounded by the Fall Line to the 

east and expanded beyond the Maryland state boundary to 

include parts of the 8-digit Hydrologic Units with rivers that 

flow through the fractured rock area of Maryland. The study 

area was further subdivided into 21,431 catchments from the 

National Hydrography Dataset Plus. Geologic, topographic, 

climatic, and soils data were compiled for each catchment 

to perform a principal components analysis and subsequent 

cluster analysis. These analyses resulted in six hydrogeologic 

regions that correspond well with previous regional frameworks and provide new information in crystalline rock areas. 

Delineation of hydrogeologic regions in the crystalline 

rock area of Maryland differed from previous frameworks 

that were based solely on physiography. Differences in 

recharge, base-flow index, and Precipitation minus Potential 

Evapotranspiration are the significant factors for this new classification. The results of this study provide a georeferenced 

hydrogeologic framework to support the Maryland 

Fractured Rock Water Supply Study being conducted by 

the U.S. Geological Survey, the Maryland Department of 

the Environment Water Supply Program, and the Maryland 

Geological Survey. 

Acknowledgments

The authors would like to thank David Wolock (USGS) 

and Mark Duigon (formerly of MGS) for their peer reviews. 

The authors would also like to thank Scott Ator, Joel 

Blomquist, and Jeff Raffensperger of the USGS for helpful 

technical discussions throughout the investigation. Thanks are 

extended to Saeid Kasraei, John Grace, and Patrick Hammond 

of the Maryland Department of the Environment Water Supply 

Program for insightful comments on an early draft of this 

report. Lastly, thanks to Timothy Auer and Valerie Gaine of 

USGS who aided in the preparation of this report. 

References Cited

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2008, Characteristics and classification of least altered 

streamflows in Massachusetts: U.S. Geological Survey 

Scientific Investigations Report 2007–5291, 113 p., plus 

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Appendix

Hydrogeologic regions (HRs) for the fractured rock 

area of Maryland and parts of adjacent states are available 

online in a comma-delimited ASCII file at http://pubs.usgs.

gov/sir/2013/5043/frac_rx_HRs.csv. Variables in the file are 

described in the header (denoted by lines starting with “#”) 

and also include principal component scores that were derived 

from principal components analysis on the 10 original input 

metrics and were used in cluster analysis to create the HRs. 

The file includes the hydrogeologic regions predictions for 

local catchments contributing to 21,431 stream reaches in 

the fractured rock area of Maryland and adjacent states as 

defined by the National Hydrography Dataset Plus (NHDPlus) 

medium resolution (1:100,000-scale) geospatial dataset 

(Horizon Systems, 2010). 
Prepared by USGS West Trenton Publishing Service Center.

Edited by Valerie M. Gaine.

Graphics and layout by Timothy W. Auer.

For additional information, contact:

Director, MD-DE-DC Water Science Center

U.S. Geological Survey

5522 Research Park Drive

Baltimore, MD 21228

or visit our Web site at:

http://md.water.usgs.gov
Fleming, B.J., and others—Statistical Classification of Hydrogeologic Regions in the Fractured Rock Area of Maryland and Parts of Adjacent States—Scientific Investigations Report 2013–5043

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