Simulation of ground-water flow in the Cedar River alluvial aquifer flow system, Cedar Rapids, Iowa

Abstract

The Cedar River alluvial aquifer is the primary source of municipal water in the Cedar Rapids, Iowa, area. Since 1992, the U.S. Geological Survey, in cooperation with the City of Cedar Rapids, has investigated the hydrogeology and water quality of the Cedar River alluvial aquifer. This report describes a detailed analysis of the groundwater flow system in the alluvial aquifer, particularly near well field areas. The groundwater flow system in the Cedar Rapids area consists of two main components, the unconsolidated Quaternary deposits and the underlying carbonate bedrock that has a variable fracture density. Quaternary deposits consist of eolian sand, loess, alluvium, and glacial till. Devonian and Silurian bedrock aquifers overlie the Maquoketa Shale (Formation) of Ordovician age, a regional confining unit. Groundwater and surface-water data were collected during the study to better define the hydrogeology of the Cedar River alluvial aquifer and Devonian and Silurian aquifers. Stream stage and discharge, groundwater levels, and estimates of aquifer hydraulic properties were used to develop a conceptual groundwater flow model and to construct and calibrate a model of the flow system. This model was used to quantify the movement of water between the various components of the alluvial aquifer flow system and provide an improved understanding of the hydrology of the alluvial aquifer. Groundwater flow was simulated for the Cedar River alluvial aquifer and the Devonian and Silurian aquifers using the three-dimensional finite-difference groundwater flow model MODFLOW. The model was discretized into 223 rows and 354 columns of cells. Areal cell sizes range from about 50 feet on a side near the Cedar River and the Cedar Rapids municipal wells to 1,500 feet on a side near the model boundaries and farthest away from the Cedar Rapids municipal well fields. The model is separated into five layers to account for the various hydrogeologic units in the model area. Model results indicate that the primary sources of inflow to the modeled area are infiltration from the Cedar River (53.0 percent) and regional flow in the glacial and bedrock materials (34.1 percent). The primary sources of outflow from the modeled area are discharge to the Cedar River (45.4 percent) and pumpage (44.8 percent). Current steady-state pumping rates have increased the flow of water from the Cedar River to the alluvial aquifer by 43.8 cubic feet per second. Steadystate and transient hypothetical pumpage scenarios were used to show the relation between changes in pumpage and changes in infiltration of water from the Cedar River. Results indicate that more than 99 percent of the water discharging from municipal wells infiltrates from the Cedar River, that the time required for induced river recharge to equilibrate with municipal pumpage may be 150 days or more, and that groundwater availability in the Cedar Rapids area will not be significantly affected by doubling current pumpage as long as there is sufficient flow in the Cedar River to provide recharge.

Selected water-quality data from the Cedar River and Cedar Rapids well fields, Cedar Rapids, Iowa, 2008–17

“…The Cedar River is in direct hydraulic connection with the alluvial aquifer (Turco and Buchmiller, 2004), and the alluvial aquifer is recharged by infiltration from the river, as well as by precipitation and seepage from underlying and adjacent hydrogeologic units. In areas affected by municipal pumping, groundwater flows from the Cedar River toward the well fields, whereas in other areas, groundwater generally flows toward the river (Hansen, 1970;Schulmeyer, 1995;R.…”

“…Water-quality data were used to assess the quality of water in the alluvial aquifer, Silurian-Devonian aquifer, and the Cedar River. The Cedar River is the primary influencer on water quality in the alluvial aquifer because of induced infiltration from the river as a result of the pumping of wells (Schulmeyer and Schnoebelen, 1998;Boyd, 2000;Turco and Buchmiller, 2004 . Water-quality data were also evaluated for nutrients, pesticides, selected major ions, selected viruses, and physical characteristics.…”

Conceptual and numerical groundwater flow model of the Cedar River alluvial aquifer system with simulation of drought stress on groundwater availability near Cedar Rapids, Iowa, for 2011 through 2013

“…In 2013, the U.S. Geological Survey (USGS), in cooperation with the City of Cedar Rapids, began a study to better understand the effects of drought stress on the Cedar River alluvial aquifer using a numerical groundwater flow model. Previously published groundwater flow models were constructed before the 2012 drought and did not simulate water level declines like those observed during the 2012 drought (Schulmeyer and Schnoebelen, 1998;Turco and Buchmiller, 2004). The USGS groundwater flow model developed during the 2013 study combined published hydrogeologic data with airborne, waterborne, down-hole, and land-based geophysical survey data collected from 2015 to 2017 that (1) provided a detailed three-dimensional lithologic model of the Cedar River alluvial aquifer and surrounding area, (2) improved the conceptual model for the groundwater flow system, and (3) evaluated hydrogeologic characteristics of aquifer materials.…”

Delineation of selected lithologic units using airborne electromagnetic data near Cedar Rapids, Iowa

“…The U.S. Geological Survey (USGS), in cooperation with the City of Cedar Rapids, began a study in 2013 to better understand the effects of drought stress on the Cedar River alluvial aquifer. Previously published groundwater-flow models provided flow analysis based on the existing understanding of the physical properties of the alluvial aquifer and the effects of pumping on source waters (Schulmeyer and Schnoebelen, 1998;Turco and Buchmiller, 2004). After an evaluation of the existing groundwater-flow models for the alluvial aquifer (Schulmeyer and Schnoebelen, 1998;Turco and Buchmiller, 2004), a plan was begun to construct an updated groundwater-flow model capable of evaluating the effects of prolonged drought and increased demand.…”

“…Previously published groundwater-flow models provided flow analysis based on the existing understanding of the physical properties of the alluvial aquifer and the effects of pumping on source waters (Schulmeyer and Schnoebelen, 1998;Turco and Buchmiller, 2004). After an evaluation of the existing groundwater-flow models for the alluvial aquifer (Schulmeyer and Schnoebelen, 1998;Turco and Buchmiller, 2004), a plan was begun to construct an updated groundwater-flow model capable of evaluating the effects of prolonged drought and increased demand. As part of the effort to update the existing groundwater-flow models, data were collected during an airborne electromagnetic (AEM) survey in May 2017 (Deszcz-Pan and others, 2018; fig.…”