Connections between Forested and Urban Landscapes and Implications for Water Supply

Science Center Objects

Interactions between forested and urban landscapes can lead to reciprocal effects that have substantial impacts on water supply and ecology. Air pollution from urban and forested landscapes can be deposited on adjacent forests, while forest disturbance, such as wildfires and floods, can remobilize those contaminants. Additionally, pollutants from legacy land use (e.g., mining) can also be mobilized to downstream urban waters through disturbance. Urban streams are subject to a complementary set of stressors including nutrient over-enrichment, dramatic daily changes in oxygen and pH, and disturbed thermal regimes. Therefore, legacy pollutants transported from the forested part of the watershed to lowland urban streams can be subject to a separate set of processes which enhances both their mobility and impacts on water quality. This project will build upon USGS research in the Colorado Front Range, a rapidly urbanizing region dependent upon forested lands for water supply, to investigate 1) the mechanisms of connection between forested and urban landscapes; and 2) how the drivers of pollutant transport differ between the two areas.

 

Fast moving stream with rapids in upper, more mountainous part of Boulder Creek Watershed, Colorado

Photograph of upper portion of Boulder Creek Watershed, CO.

(Credit: Sheila Murphy, USGS. Public domain.)

Statement of Problem: High-population areas located at the base of mountains often depend on upland forested areas to provide their water supply. These water source areas typically provide a higher quality of water than other land uses. Yet development (e.g., urbanization and agriculture) in adjacent lowlands can result in interactions that have substantial impacts on water supply and ecology. Forested land disturbances, such as wildfires and floods, can mobilize sediment, nutrients, and contaminants from legacy land use (e.g., arsenic from mining) and transport them downstream to urbanized and agricultural areas, where they are subject to in-stream processes such as oxidation-reduction reactions and conversion via biological metabolism. Conversely, atmospheric pollution from urban/agricultural areas can be transported to and deposited on upland forests, altering biogeochemical cycling (movement of chemical substances in Earth’s biotic and abiotic systems). Once deposited in upland forests, disturbances may later remobilize the contaminants.

The Colorado Front Range provides an example of these interacting landscapes. Forested uplands provide the majority of water supplies used by both agriculture and rapidly growing urban centers. However, this supply can be threatened by interacting effects of legacy pollutants and landscape disturbances. Future climate change is likely to increase disturbances in this area, and so understanding the connections between forest and developed land is of high importance.

Slow moving stream in lower, flatter part of Boulder Creek Watershed, Colorado

Photograph of lower portion of Boulder Creek Watershed, CO.

(Credit: Sheila Murphy, USGS. Public domain.)

This work builds on USGS research in the Boulder Creek watershed in the Colorado Front Range, which has investigated the roles of water management, landscape disturbance, and biogeochemical processes on water supplies (e.g., Murphy 2006; Antweiler et al. 2014; Murphy et al. 2015, 2018). The upper and lower portions of the Boulder Creek watershed have distinctly different geology, climate, land cover, and stream characteristics. The upper half is primarily secondary growth forest on granitic/metamorphic bedrock and is sparsely populated, while the lower half consists of high-density urban development and agricultural land use on sedimentary bedrock. Streams in the upper portion of the watershed are cold, fast, and have steep slopes, while those in the lower portion are warmer, more sluggish, and have shallow slopes. Streams in the lower portion represent areas of sediment deposition rather than erosion. Furthermore, urbanization, water withdrawals, and wastewater inputs create conditions of extreme biogeochemical cycling in the lower watershed. These biogeochemical cycles are likely to control the bioavailability and transport of legacy pollutants.

This project will allow better understanding of how land use and disturbance in one landscape is expressed in water quality of adjacent landscapes. It will shed light on the transfer of contaminants through stream networks and the hypothesized contrasts in mechanisms of mobilization and bioavailability in different parts of the watershed. This understanding is critical not only for the Colorado Front Range, but in many regions across the U.S., where water from forested mountains sustains population and economic growth. The work will assist land and water managers in identifying best management practices for our Nation’s natural resources. It will build on existing collaborations with the USGS Colorado Water Science Center, National Science Foundation-funded Boulder Creek Critical Zone Observatory, the University of Colorado, a local water provider (City of Boulder), the USGS Hydrologic Networks group, and the National Atmospheric Deposition Program.

 

Why this Research is Important: This project addresses critical questions regarding the interactions of land use, water management, and pollutant transport, such as how disturbances remobilize contaminants from historical land use and how that impacts water quality and water availability; how watershed management affects water quality; and how air pollution affects water quality. These questions have important implications for land use development planning, water supplies, and aquatic ecosystems. The work will assist land and water managers in identifying best management practices for our natural resources.

Map of different land cover types and burned areas in the Colorado Front Range

Map showing land cover and burned area outlines in the Colorado Front Range (modified from Murphy, 2006 and Murphy et al., 2015)

(Public domain.)

 

Objective(s): This project seeks to resolve several important questions:

  1. What are the mechanisms of connection between upper (forested) and lower (urban) watershed areas?
  2. Do the drivers of pollutant transport differ between the upper and lower watershed areas?

 

Methods: We will use field water-quality observations to demonstrate the degree of connectivity between the upper and lower watersheds. We expect similar water quality during the high flow, seasonal snowmelt period, which would indicate a high degree of connectivity within the basin. In contrast, spatially varied water quality conditions are expected at low flow and would indicate a lack of connectivity within the basin.  Daily changes in oxygen and pH will be observed using on site measurement probes. We expect large daily variability in the lower watershed, indicating a high degree of in-stream transformation of pollutants and other water quality constituents. On the other hand, the upper watershed is expected to show limited in-stream transformation and the movement of pollutants is expected to be controlled primarily by passive downstream transport during high flow periods.

 

 

 

 

 

 

 

 

 

 

References

Antweiler, R.C., Writer, J.H., and Murphy, S.F., 2014, Evaluation of wastewater contaminant transport in surface waters using verified Lagrangian sampling: Science of the Total Environment: v. 470–471, p. 551-558.

Murphy, S.F., 2006. State of the watershed: water quality of Boulder Creek, Colorado. Circular 1284, 34 p.

Murphy, S.F., McCleskey, R.B., Martin, D.A., Writer, J.H., and Ebel, B.A., 2018, Fire, flood, and drought—Extreme climate events alter flow paths and stream chemistry: Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2017JG004349.

Murphy, S.F.; Writer, J.H.; McCleskey, R.B.; Martin, D.A., 2015. The role of precipitation type, intensity, and spatial distribution in source water quality after wildfire: Environmental Research Letters, v. 10, no. 8, 084007: 1-13. doi:10.1088/1748-9326/10/8/08400713