Recharge to the Columbia River Basalt Groundwater System
Learn about the groundwater flow system within the Columbia River Basalt Group in the Umatilla River Basin.
USGS scientists used new techniques to estimate groundwater recharge in a structurally complex region underlain by rocks of the Columbia River Basalt Group (CRBG) in northeastern Oregon. The basalt in this region is heavily faulted, which led to questions about how easily groundwater can move from uplands in the Blue Mountains where precipitation amounts are largest to lowland areas where most groundwater pumping occurs.
The Confederated Tribes of the Umatilla Indian Reservation’s (CTUIR) interest in sustainable groundwater use made this research possible and the work supports ongoing Tribal water-right settlement negotiations. On the Umatilla Indian Reservation, within the Umatilla River basin, and across the larger Columbia Plateau, groundwater from the CRBG aquifer system is an important resource for drinking water and crop irrigation. Study results have potential implications for the region's population and billion-dollar annual agricultural economy.
Groundwater recharge to the CRBG occurs through precipitation, irrigation, and leakage from streams and reservoirs, with potential contributions from ancient mega floods. After entering the groundwater system, water moves through cracks and cavities in the extensive layers of basalt and can remain underground for thousands of years before eventually exiting naturally or by pumping from wells drilled into the basalt.
The age of groundwater and other isotope tracers were used to evaluate the groundwater connection between the Blue Mountain uplands, nearby foothills, and more distant lowland areas near the Columbia River. With help from CTUIR, tracer samples were collected from 34 wells, 36 springs, and 18 streams. Well locations were selected to represent different potential flow paths in the aquifer system.
Age dating groundwater samples:
Scientists can tell how old groundwater is by measuring water samples for various age “tracers”, which are dissolved chemical components from the atmosphere. The value of these components can be traced back in time to known atmospheric conditions to determine when the water entered the groundwater system. Age tracers from the atmosphere include naturally occurring isotopes, isotopes that were introduced from above-ground nuclear weapons testing in the 1950s and 1960s, and manufactured gasses. The tracers all occur at low levels and pose no threat to human health but are valuable tools for scientists seeking to understand the movement of groundwater.
All wells and several springs used in the study were sampled for various age tracers to understand groundwater movement in the CRBG aquifer system.
Result highlights:
The study identified four distinct groups of groundwater based on their stable isotope ratios, age, and relation to elevation:
1. Spring and Stream Base Flow Water: This water has spent little time underground before exiting to springs and contributing to stream base flow. Its chemistry indicates it is similar to recent rainwater and snowmelt.
2. Young Well Water: This water has a mean age of less than 3,000 years and is considered relatively young. It is chemically similar to the first group, indicating it recharged in a climate much like the one that exists today.
3. Old Well Water: This water is significantly older than the Young Well Water and entered the ground more than 12,000 years ago – before the end of the last Ice Age. This groundwater has distinct isotope patterns that indicate it recharged under cooler temperatures and has no evidence of evaporation which contrasts with the younger waters.
4. Intermediate Age Well Water: This water is between 3,000 to 12,000 years old. Groundwater mixing and climate fluctuations during this period make this a transitional group of wells with chemical components that can resemble either the Young Well Water or Old Well Water.
Groundwater movement and recharge:
Groundwater recharge to the CRBG basalt groundwater system is typically less than three millimeters per year across the study area. This slow movement is due to the thick and dense interior of basalt flows, which significantly restrict vertical water movement. The small amount of infiltrating water is concentrated in thin, generally horizontal, rubbly layers that formed the upper and lower crust of the erupting lava flows. Groundwater can move more easily through these thin, rubbly layers and it is these layers that provide water to wells drilled into the CRBG.
The recharge rates determined by this study are up to 10,000 times lower than estimates from earlier studies. The earlier recharge rates were developed using different methods, such as water-budget approaches, which may not fully capture the complexities of the CRBG groundwater system. The recharge rates determined by this study were independent of the amount of precipitation falling in different parts of the basin – recharge to wells in the Blue Mountains is similar to recharge to wells near Pendleton despite large differences in annual precipitation.
Faulting, topographic features, and the distinct permeability properties of the basalt layers effectively separate the groundwater system in the lowlands from the groundwater system in the Blue Mountains. Most recharge in the mountains exits the groundwater system rather quickly through seeps, springs, and stream base flow, with limited movement into the deep groundwater system. The limited amount of deep groundwater recharge that does occur in the uplands is restricted from moving into the lowlands due to wide and complex fault zones that separate the two regions of the Umatilla River basin.
These new recharge estimates indicate that most of the CRBG groundwater system in the Umatilla River basin is not recharged on meaningful human timescales and should be considered a finite resource. Groundwater thousands to tens of thousands of years old has been documented in many other areas of eastern Oregon and Washington where CRBG lava flows are used as a source of water. Processes similar to those observed in the Umatilla River basin may limit recharge to this larger regional groundwater system. Improving our understanding of recharge to the CRBG groundwater system will lead to better management of this important regional water resource.
Learn about the groundwater flow system within the Columbia River Basalt Group in the Umatilla River Basin.
USGS scientists used new techniques to estimate groundwater recharge in a structurally complex region underlain by rocks of the Columbia River Basalt Group (CRBG) in northeastern Oregon. The basalt in this region is heavily faulted, which led to questions about how easily groundwater can move from uplands in the Blue Mountains where precipitation amounts are largest to lowland areas where most groundwater pumping occurs.
The Confederated Tribes of the Umatilla Indian Reservation’s (CTUIR) interest in sustainable groundwater use made this research possible and the work supports ongoing Tribal water-right settlement negotiations. On the Umatilla Indian Reservation, within the Umatilla River basin, and across the larger Columbia Plateau, groundwater from the CRBG aquifer system is an important resource for drinking water and crop irrigation. Study results have potential implications for the region's population and billion-dollar annual agricultural economy.
Groundwater recharge to the CRBG occurs through precipitation, irrigation, and leakage from streams and reservoirs, with potential contributions from ancient mega floods. After entering the groundwater system, water moves through cracks and cavities in the extensive layers of basalt and can remain underground for thousands of years before eventually exiting naturally or by pumping from wells drilled into the basalt.
The age of groundwater and other isotope tracers were used to evaluate the groundwater connection between the Blue Mountain uplands, nearby foothills, and more distant lowland areas near the Columbia River. With help from CTUIR, tracer samples were collected from 34 wells, 36 springs, and 18 streams. Well locations were selected to represent different potential flow paths in the aquifer system.
Age dating groundwater samples:
Scientists can tell how old groundwater is by measuring water samples for various age “tracers”, which are dissolved chemical components from the atmosphere. The value of these components can be traced back in time to known atmospheric conditions to determine when the water entered the groundwater system. Age tracers from the atmosphere include naturally occurring isotopes, isotopes that were introduced from above-ground nuclear weapons testing in the 1950s and 1960s, and manufactured gasses. The tracers all occur at low levels and pose no threat to human health but are valuable tools for scientists seeking to understand the movement of groundwater.
All wells and several springs used in the study were sampled for various age tracers to understand groundwater movement in the CRBG aquifer system.
Result highlights:
The study identified four distinct groups of groundwater based on their stable isotope ratios, age, and relation to elevation:
1. Spring and Stream Base Flow Water: This water has spent little time underground before exiting to springs and contributing to stream base flow. Its chemistry indicates it is similar to recent rainwater and snowmelt.
2. Young Well Water: This water has a mean age of less than 3,000 years and is considered relatively young. It is chemically similar to the first group, indicating it recharged in a climate much like the one that exists today.
3. Old Well Water: This water is significantly older than the Young Well Water and entered the ground more than 12,000 years ago – before the end of the last Ice Age. This groundwater has distinct isotope patterns that indicate it recharged under cooler temperatures and has no evidence of evaporation which contrasts with the younger waters.
4. Intermediate Age Well Water: This water is between 3,000 to 12,000 years old. Groundwater mixing and climate fluctuations during this period make this a transitional group of wells with chemical components that can resemble either the Young Well Water or Old Well Water.
Groundwater movement and recharge:
Groundwater recharge to the CRBG basalt groundwater system is typically less than three millimeters per year across the study area. This slow movement is due to the thick and dense interior of basalt flows, which significantly restrict vertical water movement. The small amount of infiltrating water is concentrated in thin, generally horizontal, rubbly layers that formed the upper and lower crust of the erupting lava flows. Groundwater can move more easily through these thin, rubbly layers and it is these layers that provide water to wells drilled into the CRBG.
The recharge rates determined by this study are up to 10,000 times lower than estimates from earlier studies. The earlier recharge rates were developed using different methods, such as water-budget approaches, which may not fully capture the complexities of the CRBG groundwater system. The recharge rates determined by this study were independent of the amount of precipitation falling in different parts of the basin – recharge to wells in the Blue Mountains is similar to recharge to wells near Pendleton despite large differences in annual precipitation.
Faulting, topographic features, and the distinct permeability properties of the basalt layers effectively separate the groundwater system in the lowlands from the groundwater system in the Blue Mountains. Most recharge in the mountains exits the groundwater system rather quickly through seeps, springs, and stream base flow, with limited movement into the deep groundwater system. The limited amount of deep groundwater recharge that does occur in the uplands is restricted from moving into the lowlands due to wide and complex fault zones that separate the two regions of the Umatilla River basin.
These new recharge estimates indicate that most of the CRBG groundwater system in the Umatilla River basin is not recharged on meaningful human timescales and should be considered a finite resource. Groundwater thousands to tens of thousands of years old has been documented in many other areas of eastern Oregon and Washington where CRBG lava flows are used as a source of water. Processes similar to those observed in the Umatilla River basin may limit recharge to this larger regional groundwater system. Improving our understanding of recharge to the CRBG groundwater system will lead to better management of this important regional water resource.