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USGS2005

Numerical assessment of a landfill compliance limit

The PLASM and Random Walk ground-water flow and contaminant transport models were used to assess the potential impact of various proposed regulatory compliance distances on landfill siting. Contaminant transport modeling was performed for 16 generalized geological sequences representative of hydrogeological conditions over an estimated 90 to 95 percent of Illinois. Results of this modeling indicate that about 50 percent of the state would be hydrogeologically suitable for landfilling of nonhazardous wastes if the compliance distance was 100 feet. With a compliance distance of 500 feet, about 55 percent of the state would be hydrogeologically suitable. This work demonstrates the utility of computer modeling in the development of regulations governing landfill siting.

Groundwater↗

USGS1992

Cartographic applications software

The Office of the Assistant Division Chief for Research, National Mapping Division, develops computer software for the solution of geometronic problems in the fields of surveying, geodesy, remote sensing, and photogrammetry. Software that has been developed using public funds is available on request for a nominal charge to recover the cost of duplication.

Fact Sheet↗

USGS1992

Selected papers in the applied computer sciences 1992

This compilation of short papers reports on technical advances in the applied computer sciences. The papers describe computer applications in support of earth science investigations and research. This is the third volume in the series "Selected Papers in the Applied Computer Sciences." Listed below are the topics addressed in the compilation: Integration of geographic information systems and expert systems for resource management, Visualization of topography using digital image processing, Development of a ground-water data base for the southeastern Uited States using a geographic information system, Integration and aggregation of stream-drainage data using a geographic information system, Procedures used in production of digital geologic coverage using compact disc read-only memory (CD-ROM) technology, and Automated methods for producing a technical publication on estimated water use in the United States.

Bulletin↗

USGS2002

Assessment of undiscovered deposits of gold, silver, copper, lead, and zinc in the United States: A Portable Document (PDF) recompilation of USGS Open-File Report 96-96 and Circular 1178

This publication contains the results of a national mineral resource assessment study. The study (1) identifies regional tracts of ground believed to contain most of the nation's undiscovered resources of gold, silver, copper, lead, and zinc in conventional types of deposits; and (2) includes probabilistic estimates of the amounts of these undiscovered resources in most of the tracts. It also contains a table of the significant known deposits in the tracts, and includes descriptions of the mineral deposit models used for the assessment. The assessment was previously released in two major publications. The conterminous United States assessment was published in 1996 as USGS Open-File Report 96-96. Subsequently, the Alaska assessment was combined with the conterminous assessment in 1998 and released as USGS Circular 1178. This new recompilation was undertaken for several reasons. First, the graphical browser software used in Circular 1178 was ONLY compatible with the Microsoft Windows operating system. It was incompatible with the Macintosh operating system, Linux, and other types of Unix computers. Second, the browser on Circular 1178 is much less intuitive to operate, requiring most users to follow a tutorial to understand how to navigate the information on the CD. Third, this release corrects several errors and numbering inconsistencies in Circular 1178.

Open-File Report↗

USGS2018

Revised technical implementation plan for the ShakeAlert system—An earthquake early warning system for the West Coast of the United States

The U.S. Geological Survey (USGS), along with partner organizations, has developed an earthquake early warning (EEW) system called ShakeAlert for the highest risk areas of the United States: namely, California, Oregon, and Washington. The purpose of the system is to reduce the impact of earthquakes and save lives and property by providing alerts to institutional users and the public. Using networks of ground-motion sensors and sophisticated computer algorithms, ShakeAlert can detect an earthquake seconds after it begins, calculate its location and magnitude, and estimate the resulting intensity of shaking. Alerts can then be sent to people and systems that may experience damaging shaking, allowing them to take appropriate protective actions. Depending on the user’s distance from the earthquake, alerts may be delivered before, during, or after the arrival of strong shaking. ShakeAlert is built on the foundation of the sensor networks and data processing infrastructure of the USGS-led Advanced National Seismic System. However, these networks were not originally designed for EEW; old equipment needs to be updated and new stations must be added to construct EEW-capable networks. The ShakeAlert data-processing infrastructure includes redundant servers that are geographically distributed at monitoring centers in Seattle, Washington, as well as Menlo Park, Berkeley, and Pasadena in California. Three data-processing layers collect raw ground-motion data from field stations (data layer), analyze these data to estimate the area and intensity of the resulting shaking (production layer), and publish alert products as appropriate for end users (alert layer). The alert layer can support thousands of institutional users and alert redistributors, but the USGS does not have the mission, infrastructure, or expertise to perform public notifications and is therefore recruiting technology enablers from the private sector. Additionally, ShakeAlert will coordinate with both public and private partners to accomplish consistent and ongoing public communication, education, and outreach. The estimated cost of completing the ShakeAlert infrastructure and sensor networks is \$39.4 million and has an estimated annual operation and maintenance cost of \$28.6 million per year. Building a highly reliable data telemetry infrastructure would cost another \$20.5 million and operating this telemetry system would add \$49.8 million per year; however, these costs could be reduced if project partners provide bandwidth on existing systems.

Open-File Report↗

USGS2022

Documentation of models describing relations between continuous real-time and discrete water-quality constituents in the Little Arkansas River, south-central Kansas, 1998–2019

Data were collected at two monitoring sites along the Little Arkansas River in south-central Kansas that bracket most of the easternmost part of the Equus Beds aquifer. The data were used as part of the city of Wichita’s aquifer storage and recovery project to evaluate source water quality. The U.S. Geological Survey, in cooperation with the City of Wichita, has continued to monitor the water quality of these sites through 2019 to update previously published regression-based models using continuously measured physicochemical properties and discretely sampled water-quality constituents of interest. The purpose of this report is to provide an update of the previously published linear regression models that have been used to continuously compute estimates of water-quality constituent concentrations or densities at these two sites. Water-quality constituent model updates include those for dissolved and suspended solids, suspended-sediment concentration, hardness, alkalinity, primary ions (bicarbonate, calcium, sodium, chloride, and sulfate), nutrients (total Kjeldahl nitrogen and total phosphorus), total organic carbon, indicator bacteria ( Escherichia coli and fecal coliform bacteria), a trace element (arsenic), and a pesticide (atrazine). Regression analyses were used to develop surrogate models that related continuously measured physicochemical properties, streamflow, and seasonal components to discretely sampled water-quality constituent concentrations or densities. Specific conductance was an explanatory variable for dissolved solids, primary ions, and atrazine. Turbidity was an explanatory variable for total suspended solids and sediment, nutrients, total organic carbon, and indicator bacteria. Streamflow and water temperature were explanatory variables for dissolved arsenic. Seasonal components were included as explanatory variables for atrazine models. The amount of variance explained by most of the updated models was within 5 percent of previously published models.

Kansas↗

USGS2024

Mapping Arundo donax (Arundo cane) with multispectral imagery before, during, and after herbicide treatment along the Rio Grande in Webb County, Texas, 2020–21

Arundo donax , commonly called Arundo cane, giant reed, or Carrizo cane, is an invasive bamboo-like perennial grass common in riparian areas throughout the southwestern United States. In Texas, not only does it negatively affect riparian ecosystems, but it has also become a problem for border security because it reduces visibility along the Rio Grande. To address these problems, in 2015 the Texas State Soil and Water Conservation Board was authorized by the Texas State Legislature to develop a program to eradicate Arundo cane along the Rio Grande. In 2020, the Texas State Soil and Water Conservation Board applied imazapyr and glyphosate herbicides along a 19.3-kilometer reach of the Rio Grande, northwest of Laredo, Texas. The U.S. Geological Survey, in cooperation with the Texas State Soil and Water Conservation Board and the Webb Soil and Water Conservation District, used WorldView-3 Standard high-resolution satellite imagery to map Arundo cane extent along the reach before , during , and after the herbicide-treatment period on June 30, 2020, September 26, 2020, and May 7, 2021, respectively. A maximum likelihood supervised classification analysis was computed on the images to map the spatial extent and estimate the area covered by Arundo cane. The estimated area covered by Arundo cane in the before classification was 1,282,000 square meters, in the during classification was 1,064,000 square meters, and in the after classification was 1,108,000 square meters. The qualitative comparison of the three images shows that there was an overall decrease in vegetation classified as Arundo cane throughout the study area.

Texas↗

USGS2004

Generalized estimates from streamflow data of annual and seasonal ground-water-recharge rates for drainage basins in New Hampshire

This report presents regression equations to estimate generalized annual and seasonal ground-water-recharge rates in drainage basins in New Hampshire. The ultimate source of water for a ground-water withdrawal is aquifer recharge from a combination of precipitation on the aquifer, ground-water flow from upland basin areas, and infiltration from streambeds to the aquifer. An assessment of ground-water availability in a basin requires that recharge rates be estimated under `normal' conditions and under assumed drought conditions. Recharge equations were developed by analyzing streamflow, basin characteristics, and precipitation at 55 unregulated continuous record stream-gaging stations in New Hampshire and in adjacent states. In the initial step, streamflow records were analyzed to estimate a series of annual and seasonal ground-water-recharge components of streamflow in each drainage basin evaluated in this study. Regression equations were then developed relating the series of annual and seasonal ground-water-recharge values to the corresponding series of annual and seasonal precipitation values as determined at the centroid of each drainage basin. This resulted in one equation for each of the 55 basins for each of the four seasonal periods and the annual period, or a total of 275 regression equations. Average annual and seasonal precipitation data for 1961-90 were then used to compute a set of normalized ground-water-recharge values that reflected the long-term average annual and seasonal variations (normalized) and mean recharge characteristics of each drainage basin. Ordinary-least-squares regression was applied in the process of selecting 10 out of 93 possible basin and climatic characteristics for further testing in the development of the equations for computing the generalized estimate of annual and seasonal ground-water recharge based on the set of normalized recharge values. Generalized-least-squares regression was used for the final parameter estimation and error evaluation. The following basin and climatic characteristics were found to be statistically significant predictors for at least one of the dependent variables: average annual, summer, and spring precipitation as determined at U.S. Geological Survey stream-gaging stations; average annual basin-centroid precipitation; average mean annual basin temperature; average minimum winter basin temperature; percent coniferous forest in a basin; percent mixed coniferous and deciduous forest in a basin; average fall basin-centroid precipitation; and average annual snowcover. These 10 basin and climatic characteristics were selected because they were statistically significant based on several statistical parameters that evaluated which combination of characteristics contributed the most to the predictive accuracy of the regression-equation models. A geographic information system is required to measure the values of the predictor variables for the equations developed in the study. The average annual normalized ground-water recharge was 21.0 in. This value was determined by generalized-least-squares (GLS) regression analysis for all of the basins used in the normalized ground-water recharge analysis for precipitation from 1961-90. The average winter (January 1-March 15) ground-water recharge was 4.3 in., average spring (March 16-May 31) ground-water recharge was 9.0 in., average summer (June 1-October 31) ground-water recharge was 4.0 in., and average fall (November 1-December 31) ground-water recharge was 3.6 in. Normalized ground-water recharge ranged annually from 12.3 to 31.8 in., for winter from 2.30 to 7.82 in., for spring from 5.16 to 13.7 in., for summer from 1.45 to 10.2 in., and for fall from 2.21 to 6.06 in.

New Hampshire↗

USGS2008

Clear-Water Contraction Scour at Selected Bridge Sites in the Black Prairie Belt of the Coastal Plain in Alabama, 2006

The U.S. Geological Survey, in cooperation with the Alabama Department of Transportation, made observations of clear-water contraction scour at 25 bridge sites in the Black Prairie Belt of the Coastal Plain of Alabama. These bridge sites consisted of 54 hydraulic structures, of which 37 have measurable scour holes. Observed scour depths ranged from 1.4 to 10.4 feet. Theoretical clear-water contraction-scour depths were computed for each bridge and compared with observed scour. This comparison showed that theoretical scour depths, in general, exceeded the observed scour depths by about 475 percent. Variables determined to be important in developing scour in laboratory studies along with several other hydraulic variables were investigated to understand their influence within the Alabama field data. The strongest explanatory variables for clear-water contraction scour were channel-contraction ratio and velocity index. Envelope curves were developed relating both of these explanatory variables to observed scour. These envelope curves provide useful tools for assessing reasonable ranges of scour depth in the Black Prairie Belt of Alabama.

Scientific Investigations Report↗

USGS2018

Flood-inundation maps for Cayuga Inlet, Sixmile Creek, Cascadilla Creek, and Fall Creek at Ithaca, New York

Digital flood-inundation maps for a 2.9-square-mile area of Ithaca, New York, were created in 2015–18 by the U.S. Geological Survey in cooperation with the City of Ithaca, New York, and the New York State Department of State. The flood-inundation maps depict estimates of the maximum areal extent and depth of flooding corresponding to selected flood frequencies for Cayuga Inlet, Sixmile Creek, Cascadilla Creek, and Fall Creek and selected water-surface elevations of Cayuga Lake. Flood profiles for the stream reaches were computed by combining a one-dimensional step-backwater model for the stream channels and a two-dimensional model for the overbank areas. The resulting hydraulic model was calibrated by using water-surface profiles from five observed storm events. The model was then used to compute 15 water-surface profiles for 5 flood frequencies (50-, 10-, 2-, 1-, and 0.2-percent annual exceedance probabilities, or 2-, 10-, 50-, 100-, and 500-year recurrence intervals) and 3 lake levels (representing average conditions, a 2-year-high condition, and a 100-year-high condition). The simulated water-surface profiles were then combined with a digital elevation model (derived from light detection and ranging data having 0.31‑foot vertical accuracy and 3.3-foot horizontal resolution) to delineate the maximum area flooded at each water level. Flood-inundation maps and geographic information system flood-extent polygons and depth grids are available in the data release associated with this report. These maps can provide emergency management personnel and residents with information that is critical for flood-management planning, flood-response activities, and postflood recovery efforts.

New York↗

USGS1969

Saline water in southeastern New Mexico

Saline waters from formations of several geologic ages are being studied in a seven-county area in southeastern New Mexico and western Texas, where more than 30,000 oil and gas tests have been drilled in the past 40 years. This area of 7,500 sq. miles, which is stratigraphically complex, includes the northern and eastern margins of the Delaware Basin between the Guadalupe and Glass Mountains. Chloride-ion concentrations in water produced from rocks of various ages and depths have been mapped in Lea County, New Mexico, using machine map-plotting techniques and trend analyses. Anomalously low chloride concentrations (1,000–3,000 mg/l) were found along the western margin of the Central Basin platform in the San Andres and Capitan Limestone Formations of Permian age. These low chloride-ion concentrations may be due to preferential circulation of ground water through the more porous and permeable rocks. Data being used in the study were obtained principally from oil companies and from related service companies. The P.B.W.D.S. (Permian Basin Well Data System) scout-record magnetic-tape file was used as a framework in all computer operations. Shallow or non-oil-field water analyses acquired from state, municipal, or federal agencies were added to these data utilizing P.B.W.D.S.-compatible reference numbers and decimal latitude-longitude coordinates. Approximately 20,000 water analyses collected from over 65 sources were coded, recorded on punch cards and stored on magnetic tape for computer operations. Extensive manual and computer error checks for duplication and accuracy were made to eliminate data errors resulting from poorly located or identified samples; non-representative or contaminated samples; mistakes in coding, reproducing or key-punching; laboratory errors; and inconsistent reporting. The original 20,000 analyses considered were reduced to 6,000 representative analyses which are being used in the saline water studies.

New Mexico↗

USGS2025

Practical application of time-lapse camera imagery to develop water-level data for three hydrologic monitoring sites in Wisconsin during water year 2020

Using camera imagery to measure water level (camera-stage) is a well-researched area of study. Previous camera-stage studies have shown promising results when implementing this technology with tight constraints on test conditions. However, there is a need for a more comprehensive evaluation of the extensibility of camera-stage to practical applications. Therefore, the aim of this study was to test a camera-stage method under a wide variety of test conditions to better understand the successes and challenges of using this technology in real-world scenarios. In this study, this approach was tested during Water Year 2020 at three existing U.S. Geological Study (USGS) stream gaging stations in south central Wisconsin that had existing USGS water-level instrumentation. The specific reference objects tested were white pipes and a concrete wall. Since successful application of camera-stage relies on use of suitable images, all captured images in this study were visually inspected to determine suitability for application of camera-stage. Camera-stage measurements were then computed only on images deemed suitable and the results were compared with ground-truth stage values to determine the accuracy. For the purposes of this study, camera-stage values within ±0.10 ft of the actual stage were considered acceptable. One major challenge highlighted was the potential difficulty in obtaining suitable imagery, with the proportion of suitable images varying greatly between the four trials from 38 % to 92 %. The results from applying camera-stage to suitable images were encouraging though, with 79 % to 99 % of evaluated camera-stage values qualifying as acceptable among the four test trials.

Wisconsin↗

USGS2022

Velocity modeling of supercritical pore fluids through porous media under reservoir conditions with applications for petroleum secondary migration and carbon sequestration plumes

Computational methods to characterize secondary migration in porous media traditionally rely on fluid transport equations with assumptions of time invariance, such as flowpath modeling of buoyancy vectors, statistical percolation algorithms, capillary pressure curves, or a form of Darcy’s Law which presumes instantaneous fluid transport. However, in petroleum systems modeling, the timeframe of secondary migration from source to reservoir is important to quantify in relation to other geologic factors such as timing of petroleum generation, fault movement, and seal formation. Additionally, quantifying migration velocities enables an estimation of the distance a plume of geologically sequestered carbon dioxide travels over time, as well as the identification of low-permeability strata appropriate for long-term containment. This study introduces a method to quantify transport velocities of supercritical fluids in low-permeability lithologies for a broad range of rock and fluid properties likely encountered in the sedimentary sequence. A time-dependent form of Darcy’s Law for pressure-driven viscous flow through homogeneous isotropic porous media was used to model flow velocities within a carrier bed. Thermodynamic equations of state were used to determine thermophysical properties of supercritical pore fluids under reservoir pressures ranging from 0–200 MPa (0–29,000 psi) to constrain the momentum equations. Three case studies were examined that (1) estimated fluid flow velocities of methane within the low-permeability Upper Jurassic Haynesville Formation, (2) defined permeability-based flow units to evaluate saline formations for long-term geologic carbon sequestration, and (3) calculated the migration distance of carbon dioxide plumes at the Decatur, Illinois injection and sequestration project.

SEG-AAPG Interpretation↗

USGS2017

Virginia flow-ecology modeling results—An initial assessment of flow reduction effects on aquatic biota

Background The U.S. Geological Survey (USGS), in cooperation with the Virginia Department of Environmental Quality (DEQ), reviewed a previously compiled set of linear regression models to assess their utility in defining the response of the aquatic biological community to streamflow depletion. As part of the 2012 Virginia Healthy Watersheds Initiative (HWI) study conducted by Tetra Tech, Inc., for the U.S. Environmental Protection Agency (EPA) and Virginia DEQ, a database with computed values of 72 hydrologic metrics, or indicators of hydrologic alteration (IHA), 37 fish metrics, and 64 benthic invertebrate metrics was compiled and quality assured. Hydrologic alteration was represented by simulation of streamflow record for a pre-water-withdrawal condition (baseline) without dams or developed land, compared to the simulated recent-flow condition (2008 withdrawal simulation) including dams and altered landscape to calculate a percent alteration of flow. Biological samples representing the existing populations represent a range of alteration in the biological community today. For this study, all 72 IHA metrics, which included more than 7,272 linear regression models, were considered. This extensive dataset provided the opportunity for hypothesis testing and prioritization of flow-ecology relations that have the potential to explain the effect(s) of hydrologic alteration on biological metrics in Virginia streams.

Virginia↗

USGS2020

Time-series model, statistical methods, and software documentation for R–QWTREND—An R package for analyzing trends in stream-water quality

As part of a U.S. Geological Survey water-quality study started in 2018, in cooperation with the International Joint Commission, North Dakota Department of Environmental Quality, and Minnesota Pollution Control Agency, a publicly available software package called R–QWTREND was developed for analyzing trends in stream-water quality. The R–QWTREND package is a collection of functions written in R, an open source language and a general environment for statistical computing and graphics. The package uses a parametric time-series model to express logarithmically transformed concentration in terms of flow-related variability, trend, and serially correlated model errors. Flow-related variability captures natural variability in concentration on the basis of concurrent and antecedent streamflow. The trend identifies systematic changes in concentration in terms of potential step trends, piecewise monotonic trends, or user-specified trends. Maximum likelihood estimation is used to estimate model parameters and determine the best-fit trend model. This report describes the time-series model and statistical methodology behind R–QWTREND and provides formal documentation for installing and using the package.

Open-File Report↗

USGS2002

Occurrence and status of volatile organic compounds in ground water from rural, untreated, self-supplied domestic wells in the United States, 1986-99

Samples of untreated ground water from 1,926 rural, self-supplied domestic wells were analyzed for volatile organic compounds (VOCs) during 1986-99. This information was used to characterize the occurrence and status of VOCs in domestic well water. The samples were either collected as part of the U.S. Geological Survey?s National Water-Quality Assessment (NAWQA) Program occurrence-assessment studies or were compiled by NAWQA from existing ambient ground-water or source-water-quality monitoring programs conducted by local, State, and other Federal agencies. Water samples were collected at the wellhead prior to treatment or storage. In most samples, 55 target VOCs were analyzed, and occurrence and status information generally was computed at an assessment level of 0.2 mg/L (microgram per liter). At least one VOC was detected in 12 percent of samples (232 samples) at an assessment level of 0.2 mg/L. This detection frequency is relatively low compared to the 26 percent detection frequency of at least one VOC in public sup-ply wells sampled by NAWQA, and the difference may be due, in part, to the higher pumping rates, pumping stress factors, and larger contributing areas of public supply wells. Samples with detections of at least one VOC were collected from wells located in 31 of 39 States. Solvents were the most frequently detected VOC group with detections in 4.6 percent of samples (89 samples) at an assessment level of 0.2 mg/L. The geographic distribution of detections of some VOC groups, such as fumigants and oxygenates, relates to the use pattern of com-pounds in that group. With the exception of com-pounds used in organic synthesis, detection frequencies of VOCs by group are proportional to the average half-life of compounds in the group. When the organic synthesis group is excluded from the analysis, a good correlation exists between the detection frequency of VOCs by group and average half-life of compounds in the group. Individually, VOCs were not commonly detected at an assessment level of 0.2 mg/L, with the seven most frequently detected VOCs found in only 1 to 5 percent of samples. Mixtures (two or more compounds) were a common mode of occurrence for VOCs when no assessment level was applied, and mixtures occurred in one-half of all samples that contained at least one VOC. Only 1.4 percent of samples (27 samples) had one or more VOC concentrations that exceeded a federally established drinking-water standard or health criterion. Only 0.1 percent of samples (2 samples) had one or more VOC concentrations that exceeded a taste/odor threshold. Potential point sources of VOCs near domestic wells are numerous. Leaks from under-ground storage tanks and aboveground storage tanks that hold gasoline, diesel fuel, or heating oil have the potential to be major point sources of contaminants to domestic wells. Shock chlorination may be a source of trichloromethane and other trihalomethanes in some domestic wells. Septic systems are believed to be an important source of contaminants to domestic wells, but extensive research on this subject does not exist. VOCs frequently are ingredients in household products such as cleansers and insecticides, and some VOCs have been found in septic systems.

Water-Resources Investigations Report↗

USGS2010

Importance of record length with respect to estimating the 1-percent chance flood

U.S. Geological Survey (USGS) streamflow gages have been established in every State in the Nation, Puerto Rico, and the Trust Territory of the Pacific Islands. From these st reamflow records, estimates of the magnitude and frequency of floods are often developed and used to design transportation and water- conveyance structures to protect lives and property, and to determine flood-insurance rates. Probably the most recognizable flood statistic computed from USGS stream gaging records is the 1- percent (%) chance flood; better known has the 100-year flood. By definition, this is a flood that has a 1% chance of occurring in any given year. The 1% chance flood is a statistical estimate that can be significantly influenced by length of record and extreme flood events captured in that record. Consequently, it is typically recommended that flood statistics be updated on some regular interval such as every 10 years. This paper examines the influence of record length on the 1% chance flood for the Broad River in Georgia and the substantial difference that can occur in the estimate based on record length and the hydrologic conditions under which that record was collected.

Conference Paper↗

USGS1959

The relation between fresh and salty ground water in southern Nassau and southeastern Queens counties, Long Island, New York

An investigation of the geology and ground - water conditions in a part of southwestern Long Island , including the construction of 8 deep and 5 shallow test and observation wells, has been completed by the Geological Survey in cooperation with the Nassau County Department of Public Works and the New York State Water Power and Control Commission. In this area are four main water -bearing units of Late Cretaceous and Pleistocene age. Ground water having a higher than normal salt content is encountered in some wells that penetrate the upper three units. Although the presence of this water constitutes a potential threat, existing fresh - water supplies apparently are in little immediate danger under present conditions. The " salty " ground water has chloride concentrations ranging from about 40 to 18,000 ppm (parts per million), generally more than about 2,000 ppm. It occurs in several bodies, seaward of and beneath the fresh water in the respective aquifers, and is in contact with the fresh water through zones of diffusion. The largest salt- water body, confined under artesian pressure, is in the Jameco gravel and the Magothy (?) formation beneath extreme southwestern Nassau County and adjoining southeastern Queens County . This body is thought to have occupied a substantial part of its present extent since at least 1895. It probably has been and is now encroaching, very slowly, because of increased pumping from the fresh - water bodies and possibly as a result of a postglacial rise of sea level. The rate of encroachment under present conditions probably is less than 100 feet per year. Application of the Ghyben-Herzberg principle to compute depths to the contact between salty and fresh ground water was found to give erroneous results. More accurate depths and a clearer understanding of the relation between heads in adjoining bodies of fresh and salty ground water are obtained by use of a formula adapted from one given by M. King Hubbert.

New York↗