Characterization and occurrence of confined and unconfined aquifers in Quaternary sediments in the glaciated conterminous United States
Characterization and occurrence of confined and unconfined aquifers in Quaternary sediments in the glaciated conterminous United States
复制标题
美国冰川地区第四纪沉积物中承压和非承压含水层的特征和产状
DOI：
10.3133/sir20185091
复制
发表时间：
2019
期刊：
影响因子：
--
通讯作者：
James E. Reddy
中科院分区：
文献类型：
--
作者：
R. M. Yager;L. J. Kauffman;D. Soller;A. Haj;P. M. Heisig;C. A. Buchwald;S. Westenbroek;James E. Reddy
冰川含水层系统是美国冰川地带第四纪沉积物内的含水层的集合，是供应地下水的主要含水层，为约 4200 万人提供服务，约占美国饮用水的 5%。该含水层系统（最大冰川推进区域）位于 25 个州的部分地区之下，面积为 1.87×106 平方公里。提出了一个水文地质框架，该框架使用基于先前测绘的地质方法将冰川覆盖的美国划分为 17 个不同的水文地质地体。每个水文地质地体都含有第四纪沉积物，这些沉积物源自共同的沉积历史，并且具有相似的纹理和厚度。第四纪沉积物的特征是使用根据 24 个州（不包括肯塔基州）编制的测井岩性数据库计算得出的属性来描述的。水文地质框架呈现了冰川含水层系统的全国性图景，并提供了有关其中含水层性质的概括（例如，含水层是浅层还是深层，以及无约束还是约束）。通过这种方式，可以通过了解冰川含水层系统不同部分的相似性和差异以及它们与水的利用和质量以及含水层脆弱性的关系来获得见解。水文地质地体的描绘是基于对第四纪沉积物现有地质图和松散物质厚度的解释。第四纪沉积物的总厚度用于对每个地体中水文地质框架的广义复杂性进行定性排名：“较低”复杂性（分配地体代码 1）、“中等”复杂性（地体代码 2）和“较高”复杂性（地体代码 3）。地形代码后面附加的字母名称（例如 1A、1B 或 1C）可区分相似复杂性的地形。两个独特的区域以厚的、分层的、粗粒沉积物为主，被指定为地体代码 4。该水文地质框架的要素包括冰川环境和地表沉积物地理数据库，其中包括基于先前测绘的第四纪沉积物的岩性、地貌和地层特征；根据水井钻探记录获得的岩性测井数据计算出沉积物和含水层特征的网格数据库；用水数据库，其中包含公共供水系统和地下水源的信息；以及根据地质土壤水平衡模型计算出的估计补给量。还包括基于以前的州级测绘的通用基岩地质图。美国冰川期的第四纪沉积物包括冰川沉积物、冰后沉积物（全新世）和非冰川沉积物。在陆地表面，60%的冰川沉积物是耕地。大面积的冲刷和冰接触沉积物遍布整个中西部，但通常仅限于东北部和西北部的山谷。湖泊沉积物沉积在毗邻现今五大湖的前冰期湖泊以及达科他州东部和明尼苏达州西北部的阿加西冰湖中。 17个水文地质地体第四纪沉积物平均厚度为6米至45米，但部分地区最大厚度超过500米。第四纪沉积物通常含有少于 10% 的粗物质；在冰接触和冲刷沉积物下，中值范围接近 0%，到大约 50%。大约 80% 的粗物质位于陆地表面 25 至 40 米范围内。在美国大部分冰川地区，穿透含有至少 3 米厚的粗物质的含水层物质层的可能性很小。约 44% 的岩性测井记录了单个含水层物质层段，而约 11% 的测井记录穿透了多个层段。岩性数据库中约 44% 的水井是在第四纪沉积物中完成的，其中许多第四纪水井 (42%) 都被至少 7.5 米的细物质所限制。这些第四纪水井中约有 33% 是无约束的，其余的则仅存在薄层（小于 3 米）的粗物质。 17个水文地质地体中第四纪水井平均深度为13米至40米。充填量范围从东北部每年超过 400 毫米到达科他州和蒙大拿州每年 11 毫米或更少（中值为每年 136 毫米）。美国冰河沿岸第四纪沉积物中第 2 县承压和非承压含水层的年度地下水开采量按面积计算，范围从每年不到 1 毫米到 370 毫米不等，平均值为每年 7.4 毫米。大约 36% 的抽取量用于公共供水，其中 70% 来自第四纪沉积物。在美国大部分冰川覆盖地区，地下水抽取量不足补给量的 10%，但在东北部和中西部城市地区附近以及中西部干旱地区的各县，地下水抽取量占补给量的比例较大。 17 个水文地质地体的显着特征通过地图和一组描述性绘图呈现，以便于对选定的沉积物和含水层特征进行视觉比较。第四纪沉积物的厚度通常从较低复杂性地体到较高复杂性地体逐渐增加，这与它们的轮廓一致。第四纪沉积物中粗物质的中值比例以及含水层物质区间的深度变化很大（分别小于 10% 至 50% 和 0 至 30 米）。含水层-物质层段的中位厚度通常落在一个狭窄的范围内（10 至 20 米），除了含有厚厚的粗粒沉积物的两个地体（30 至 35 米）之外。井中的水源各不相同，从复杂性较低的地体中主要是基岩井到沉积物最厚的较高复杂性地体中的主要是第四纪井。根据基于沉积物和含水层特征的指标以及每个地体中水井的分布组成的矩阵的层次聚类分析编制的树图，表明在分析地下水流量和水质时可以将某些地体组视为可比较的。根据岩性测井绘制的地图上显示的含水层物质间隔（包括无约束和约束条件）与伊利诺伊州、印第安纳州和北达科他州地图上描绘的含水层系统很好地相关。然而，该研究的大范围限制了地图解释的分辨率，并且东北部和西北部的一些山谷的冲积单元没有正确绘制。研究中使用的岩性测井偏向于浅深度，因为并非所有测井都穿透第四纪沉积物的整个厚度，但这种偏向不应限制沉积物和含水层描述的实用性，因为浅深度通常用于供水。水文地质框架将支持美国地质调查局针对美国冰川地区的国家水质评估计划正在进行的地下水流量和质量研究。
The glacial aquifer system, which is a collection of aquifers within Quaternary sediments in the glaciated conterminous United States, is a principal aquifer that supplies groundwater that serves about 42 million people and accounts for about 5 percent of the Nation’s drinking water. This aquifer system (the area of maximum glacial advance) underlies parts of 25 States and covers 1.87×106 square kilometers. A hydrogeologic framework is presented that divides the glaciated United States into 17 distinct hydrogeologic terranes using a geologic approach based on previous mapping. Each hydrogeologic terrane contains Quaternary sediment that is derived from a common depositional history and can be characterized by similar texture and thickness. Characteristics of Quaternary sediments are described using attributes computed from a lithologic database of well logs compiled from 24 States (excluding Kentucky). The hydrogeologic framework presents a nationwide picture of the glacial aquifer system and provides generalizations concerning the nature of aquifers within it (for example, whether the aquifers are shallow or deep, and unconfined or confined). In this way insights can be gained from understanding the similarities and differences in distinct parts of the glacial aquifer system and how they relate to water use and quality and to aquifer vulnerability. Delineation of hydrogeologic terranes was based on an interpretation of existing geologic mapping of Quaternary sediments and the thickness of unconsolidated material. Overall thickness of Quaternary sediment was used to qualitatively rank the generalized complexity of the hydrogeologic framework in each terrane: “lower” complexity (assigned a terrane code 1), “moderate” complexity (terrane code 2), and “higher” complexity (terrane code 3). Letter designations appended to the terrane codes (for example, 1A, 1B, or 1C) differentiate terranes of similar complexity. Two unique areas, where thick, stratified, coarse-grained sediment dominates, were assigned terrane code 4. Elements of this hydrogeologic framework include a glacial environments and surficial sediments geodatabase, which includes lithologic, geomorphic, and stratigraphic characterization of Quaternary sediments based on previous mapping; a gridded database of sediment and aquifer characteristics computed from lithologic logs obtained from waterwell driller records; a water-use database with information on public-water supply systems and sources of groundwater; and estimated recharge computed from a geologically based soilwater balance model. A generalized map of the bedrock geology based on previous State-level mapping is included as well. Quaternary sediment in the glaciated United States includes glacial, postglacial (Holocene) and nonglacial sediments. At land surface, 60 percent of the glacial sediment is till. Large areas of outwash and ice contact sediments are extensive throughout the Midwest but generally are confined to valleys in the Northeast and the Northwest. Lacustrine sediments were deposited in proglacial lakes adjacent to the present Great Lakes and in glacial Lake Agassiz in the eastern Dakotas and northwestern Minnesota. The median thickness of Quaternary sediment ranges from 6 to 45 meters across the 17 hydrogeologic terranes, but the maximum thickness is more than 500 meters in some areas. Quaternary sediments generally contain less than 10 percent coarse material; the median range is near zero percent under till to about 50 percent under ice contact and outwash sediments. About 80 percent of the coarse material lies within 25 to 40 meters of land surface. In most of the glaciated United States, there is a small likelihood of penetrating an aquifer-material interval containing coarse material at least 3 meters thick. A single aquifermaterial interval was recorded in about 44 percent of lithologic logs, whereas about 11 percent of the logs penetrated multiple intervals. About 44 percent of water wells in the lithologic database are completed in Quaternary sediment, and many of these Quaternary water wells (42 percent) are confined by at least 7.5 meters of fine materials. About 33 percent of these Quaternary water wells are unconfined—the remainder are where only thin layers (less than 3 meters) of coarse material are present. The median depths of Quaternary water wells range from 13 to 40 meters among the 17 hydrogeologic terranes. Recharge ranges from more than 400 millimeters per year in the Northeast to 11 millimeters or less per year in the Dakotas and Montana (median value of 136 millimeters per year). Annual groundwater withdrawals compiled by county 2 Confined and Unconfined Aquifers in Quaternary Sediments in the Glaciated Conterminous United States range on an areal basis from less than 1 to 370 millimeters per year, and the mean is 7.4 millimeters per year. About 36 percent of the withdrawals are for public-water supply, of which 70 percent are derived from Quaternary sediments. Groundwater withdrawals are less than 10 percent of recharge throughout most of the glaciated conterminous United States, but are a larger proportion of recharge near urban areas in the Northeast and the Midwest, and in counties throughout drier parts of the Midwest. The salient characteristics of the 17 hydrogeologic terranes are presented through maps and a set of descriptive plots to facilitate visual comparisons between selected sediment and aquifer characteristics. The thickness of Quaternary sediment generally increases from the lower complexity terranes through the higher complexity terranes, consistent with their delineation. Median proportions of coarse material in Quaternary sediment and depths to aquifer-material intervals are highly variable (less than 10 to 50 percent, and 0 to 30 meters, respectively). Median thicknesses of aquifer-material intervals generally fall within a narrow range (10 to 20 meters), except in two terranes that contain thick coarse-grained sediment (30 to 35 meters). The source of water in wells varies from mostly bedrock wells in the lower complexity terranes to mostly Quaternary wells in the higher complexity terranes where the sediment is thickest. A tree diagram compiled from a hierarchical cluster analysis of a matrix composed of metrics based on sediment and aquifer characteristics, and the distribution of water wells in each terrane, indicates some groups of terranes that can be treated as comparable when analyzing groundwater flow and quality. Aquifer-material intervals indicated on maps prepared from the lithologic logs, including unconfined and confined conditions, correlate well with aquifer systems delineated on state maps for Illinois, Indiana, and North Dakota. The large scale of the study limits the resolution at which the maps can be interpreted, however, and alluvial units are not mapped correctly for some valleys in the Northeast and the Northwest. Lithologic logs used in the study are biased toward shallow depths because not all logs penetrate the entire thickness of Quaternary sediment, but this bias should not limit the utility of the sediment and aquifer descriptions because shallow depths are commonly exploited for water supply. The hydrogeologic framework will support ongoing studies of groundwater flow and quality in the U.S. Geological Survey National Water Quality Assessment program for the glaciated United States.