U.S. Geological Survey 

U.S. Department of the Interior

This Fact Sheet provides information on a digital data set that contains 

generalized lithogeochemical characteristics of near-surface bedrock in 

the New England Coastal Basins 

study area of the U.S. Geological 

Survey’s National Water Quality 

Assessment (NAWQA) Program. It 

can be used in a geographic information system (GIS) to analyze and interpret water-quality and ecosystem 

conditions. For example, it has been 

used to characterize water quality in 

bedrock aquifers, to determine strategies for monitoring ground-water 

quality, and to characterize watersheds for surface-water and biological studies. The digital data set, the 

associated metadata, and the related 

files are available on the World Wide 

Web at http://water.usgs.gov/pubs/of/

ofr02-007

Introduction

As part of the U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) Program, water quality 

in the New England Coastal Basins 

(NECB) study area is being characterized. Surface water, ground water, and 

aquatic biota are being studied in a 

23,000 square-mile area in western and 

central Maine, eastern Massachusetts, 

most of Rhode Island, eastern New 

Hampshire, and a small part of eastern 

Connecticut (fig. 1). Major drainage 

basins in the NECB include the 

Kennebec, Androscoggin, Saco, Merrimack, Charles, and Blackstone Rivers 

(Flanagan and others, 1999). As part of 

the study, a digital data set has been created that classifies the bedrock units of 

the NECB study on the basis of similar 

Digital Data Set of Generalized Lithogeochemical 

Characteristics of Near-Surface Bedrock in the 

New England Coastal Basins

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

USGS Fact Sheet 003-02 

March 2002

Figure 1. Areal distribution and description of the major groups of lithogeochemical 

units defined for the New England Coastal Basins study area.

science for a changing world
lithology and mineralogy for use in the 

interpretation of water-quality data. This 

Fact Sheet describes the geologic data 

sources that were compiled and the basis 

of the classification used in the digital 

data set. This Fact Sheet also describes 

the available formats of the digital data 

set and provides information on where to 

obtain these data on the World Wide 

Web.

The scheme for classifying bedrock 

units is based on geochemical principles, 

previous studies of the relations between 

water quality, ecosystem characteristics, 

and rock type, and the regional geology 

of New England. This classification 

scheme was first developed as part of the 

NAWQA study of the Connecticut, 

Housatonic, and Thames River Basins 

(CONN), an adjacent study area (fig. 1) 

(Robinson and others, 1999). Lithogeochemical characteristics of bedrock 

units were used by the NECB study to 

identify differences in the chemical quality of water from various bedrock aquifers (Ayotte and others, 1999) to 

determine strategies for monitoring 

ground-water quality in bedrock and 

surficial aquifers, and to characterize 

watersheds for surface-water and biological studies.

The chemical composition of surface 

water and ground water depends on a 

variety of climatic, geologic, geochemical, biochemical, anthropogenic, and 

hydrologic factors. Primary sources of 

most constituents in natural waters, however, are the minerals in sediments and 

rocks. Through the process of dissolution and weathering, minerals dissolve in 

water and alter its chemistry. Although 

the relation between these processes and 

other factors is complex, in similar climates, natural waters in contact with 

similar rock types often show similar 

chemical characteristics (Robinson, 

1997). For the classification system used 

by the NECB study, more than 400 previously mapped bedrock units have been 

grouped into 38 lithogeochemical units, 

which are groups of bedrock units with 

similar mineral composition, texture and 

for some bedrock units, relative age. 

The lithogeochemical units have then 

been generalized into seven major 

groups (fig. 1), which are groups of 

lithogeochemical units that share similar 

geochemical and lithological features 

(fig. 2). 

Compilation of Geologic Data 

Sources

The lithogeochemical data were 

derived and compiled from the five individual State geologic maps in the NECB 

study area (Osberg and others, 1985; 

Lyons and others, 1997; Zen and others, 

1983; Hermes and others, 1994; and

Rogers, 1985). Digital versions of the 

State geologic maps were stored in a 

geographic information system (GIS) so 

the bedrock units could be spatially 

grouped into lithogeochemical units and 

major groups. A lithogeochemical unit 

code, major group code, NAWQA-unit 

code, and State abbreviation were 

assigned to each bedrock unit in the digital State geologic maps (table 1). The 

digital State geologic maps were joined 

in the NECB study area so the lithogeochemical digital data set could be 

created. A list of bedrock units from the 

individual State geologic maps that were 

classified into each lithogeochemical 

unit and each major group is provided in 

Robinson and others (2002, appendix A) 

and is available on the World Wide Web 

at http://water.usgs.gov/pubs/of/

ofr02-007.

Because each State bedrock geologic 

map was interpreted and assembled by 

different groups of geologists, the bedrock maps are not consistent from state 

to state. In particular, there are variations in the scales of the State bedrock 

maps, the descriptions of mineral 

assemblages in the formations, and the 

regional trends in lithology and metamorphic grade. These inconsistencies 

were minimized by classifying bedrock 

units into regional lithogeochemical 

units and major groups as shown in 

figure 1.

Classification Scheme

This regional classification scheme 

groups bedrock units by major mineral 

and textural properties and by relative 

age for some bedrock units. This 

scheme is modified from the classification scheme described in Robinson and 

others (1999). The 38 lithogeochemical 

units defined for the NECB study area 

consider the combined effects of lithology, metamorphic grade, and geologic 

setting on water chemistry. Because 

some of these factors have more effect 

on water chemistry than others, some of 

the lithogeochemical units were subdivided into additional units with modifying attributes. For example, lithogeochemical units containing carbonate 

and sulfide minerals were subdivided 

because these highly reactive minerals 

have more effect on water chemistry than 

other minerals in the rocks of this region. 

Lithogeochemical units containing granitic units were subdivided to indicate 

relative age. This subdivision of granitic 

units represents a modification from the 

classification scheme described by 

Robinson and others (1999). A description of the lithogeochemical units and 

their potential effects on water quality 

can be found in Robinson and others 

(2002).

 

Table 1. Descriptions of attributes in the lithogeochemical digital data set for the 

New England Coastal Basins study area

Attribute Coverage 

abbreviation Description

Bedrock Bedrock This attribute identifies the predominant bedrock type in 

the delineated areas, and is included to help identify 

source materials for the line work and geologic 

information. It is an alphabetic and numeric code.

Lithogeochemical unitLitho_unit This attribute represents the grouping of bedrock units that 

are lithologically and geochemically similar. It is a 

numeric and alphabetic code.

Major group Maj_group This attribute represents a classification of the 

lithogeochemical units into broad groups. It is a singledigit code. 

NAWQA unit NAWQA-unit This attribute identifies New England Coastal Basins 

study area as the National Water Quality Assessment 

Program study unit associated with this digital data set.

State State This attribute identifies the State location of each polygon, 

and is included to facilitate identification of source 

materials for linework and geologic information.
Figure 2. Example application of the classification scheme that groups more than 400 bedrock map units defined in the New England Coastal Basin 

study area into 7 major groups (fig. 1) on the basis of similar lithology, mineralogy, and chemistry.
Creating a New England Regional Lithogeochemical Digital Data Set

The NECB and CONN study-unit lithogeochemical data sets can be combined to 

create a regional product (fig. 1). Both data sets were prepared following the same 

basic principles defined by Robinson (1997). When the two digital data sets are 

plotted side by side there are some notable discrepancies along the study unit 

boundaries between bedrock lines, lithogeochemical units, and major groups. 

These discrepancies are the result of modifications to the NECB data set (see the 

Classification Scheme section), differences in the digital-source materials for New 

Hampshire and Massachusetts, and differences in the bedrock units between the 

two study areas. The CONN data set (Robinson and others, 1999) can be obtained 

on the World Wide Web at http://water.usgs.gov/pubs/wri/wri994000/.

The 38 lithogeochemical units are 

generalized into 7 major groups (fig. 1) 

that share similarities in overall 

geochemistry and lithology—(1) carbonate-bearing metasedimentary rocks; 

(2) primarily noncalcareous, clastic sedimentary rocks with restricted deposition 

in discrete fault-bounded sedimentary 

basins of Mississippian or younger age; 

(3) primarily noncalcareous, clastic sedimentary rocks at or above biotite-grade 

of regional metamorphism; (4) mafic 

igneous rocks and their metamorphic 

equivalents; (5) ultramafic rocks; (6) felsic igneous rocks and their metamorphic 

equivalents; and (7) unconsolidated and 

poorly consolidated sediments. Major 

group 7 includes areas in the southcoastal part of the NECB study area 

where the bedrock is overlain by thick 

glacial sediments at the surface. These 

surficial glacial deposits are the primary 

aquifers for these areas. An example of 

how bedrock units were classified or 

grouped into lithogeochemical units and 

then into major groups is shown in 

figure 2 for a small area of New Hampshire.

Digital Products

A digital data set that contains the 

major groups, lithogeochemical units, 

and bedrock units for the NECB study 

area is available on the World Wide Web 

at URL http://water.usgs.gov/pubs/of/

ofr02-007. These data can be retrieved 

as an ArcView shape file or an ARC/

INFO export file. The metadata (documentation) of the NECB lithogeochemical data set also is available with the 

digital data set. The metadata provides 

detailed information on data quality, spatial-data organization, spatial reference, 

spatial entities and attributes, and other 

aspects of the data set. Any use of trade, 

product, or firm names is for descriptive 

purposes only and does not imply 

endorsement by the U.S. Government.

For more information on the NECB 

study and the NAWQA program, 

please contact:

New England Coastal Basins

NAWQA study

U.S. Geological Survey

361 Commerce Way

Pembroke, NH 03275

(603) 226-7809

E-mail: 

GS-W_NAWQA_NECB@usgs.gov

Web Site: http://nh.water.usgs.gov

Chief, NAWQA Program

U.S. Geological Survey

12201 Sunrise Valley Drive

National Center, MS 413

Reston, VA 20192

E-mail: nawqa_whq@usgs.gov

Web Site: http://water.usgs.gov/

nawqa/

References Cited

Ayotte, J.P., Nielsen, M.G., 

Robinson, G.R., Jr., and 

Moore, R.B., 1999, Relation of 

arsenic, iron, and manganese in 

ground water to aquifer type, bedrock lithogeochemistry, and land use 

in the New England Coastal Basins: 

U.S. Geological Survey WaterResources Investigations Report 

99-4162, 61 p. 

Hermes, O.D., Gromet, L.P., and 

Murray, D.P., 1994, Bedrock geologic map of Rhode Island: Kingston, R.I., Office of the Rhode Island 

State Geologist, Rhode Island Map 

Series no. 1, 1 map sheet, 1:100,000. 

Flanagan, S.M., Nielsen, M.G., 

Robinson, K.W., and Coles, J.F., 

1999, Water-quality assessment of 

the New England Coastal Basins in 

Maine, Massachusetts, New Hampshire, and Rhode Island: Environmental settings and implications for 

water quality and aquatic biota: 

U.S. Geological Survey WaterResources Investigations Report 

98-4249, 62 p.

Lyons, J.B., Bothner, W.A., 

Moench, R.H., and Thompson, J.B., 

Jr., 1997, Bedrock geologic map of 

New Hampshire: U.S. Geological 

Survey Special Map, 2 map sheets, 

1:250,000.

Osberg, P.H., Hussey, A.M. II, and 

Boone, G.M., 1985, Bedrock geologic map of Maine: Augusta, 

Maine, Maine Geological Survey, 

1 map sheet, 1:500,000.

Robinson, G.R., Jr., 1997, Portraying 

chemical properties of bedrock for 

water quality and ecosystem analysis: An approach for the New 

England Region: U.S. Geological 

Survey Open-File Report 97-154, 

17 p.

Robinson, G.R., Jr., Ayotte, J.P., 

Montgomery, D.L., and 

DeSimone, L.A., 2002, Lithogeochemical character of near-surface bedrock in the New England 

Coastal Basins: U.S. Geological 

Survey Open-File Report 02-007 

(digital), accessed December 2001, 

at URL http://water.usgs.gov/lookup/

getspatial?ofr02-007_lithogeo.

Robinson, G.R., Jr., Peper, J.D., 

Steeves, P.A., and DeSimone, L.A., 

1999, Lithogeochemical character of 

near-surface bedrock in the Connecticut, Housatonic, and Thames River 

Basins: U.S. Geological Survey 

Water-Resources Investigations 

Report 99-4000 (digital), accessed 

April 26, 2001, at URL http://

water.usgs.gov/lookup/

getspatial?wri99-4000_lithogeo.

Rogers, John (compiler), 1985, Bedrock 

geological map of Connecticut: 

Connecticut Geologic and Natural 

History Survey, 2 map sheets, 

1:125,000.

Zen, E-an, Goldsmith, G.R., 

Ratcliffe, N.L., Robinson, Peter, and 

Stanley, R.S., 1983, Bedrock geologic map of Massachusetts: U.S. 

Geological Survey, Monograph 

series, 3 map sheets, 1:250,000.

—By Denise L. Montgomery, 

Gilpin R. Robinson, Jr., Joseph D. Ayotte, 

Sarah M. Flanagan, and 

Keith W. Robinson