Identifying wastewater management tradeoffs: Costs, nearshore water quality, and implications for marine coastal ecosystems in Kona, Hawai'i.

Untreated and minimally treated wastewater discharged into the environment have the potential to adversely affect groundwater dependent ecosystems and nearshore marine health. Addressing this issue requires a systems approach that links land use and wastewater management decisions to potential impacts on the nearshore marine environment via changes in water quality and quantity. To that end, a framework was developed to assess decisions that have cascading effects across multiple elements of the ridge-to-reef system. In an application to Kona (Hawai'i, USA), eight land use and wastewater management scenarios were evaluated in terms of wastewater system upgrade costs and wastewater related nutrient loads in groundwater, which eventually discharge to nearshore waters, resulting in potential impacts to marine habitat quality. Without any upgrades of cesspools or the existing wastewater treatment plant (WWTP), discharges of nutrients are expected to increase substantially with permitted development, with potential detrimental impacts to the marine environment. Results suggest that converting all of the existing cesspools to aerobic treatment units (ATU) and upgrading the existing WWTP to R-1 quality provide the highest protection to nearshore marine habitat at a cost of $569 million in present value terms. Other wastewater management options were less effective but also less costly. For example, targeted cesspool conversion (a combination of septic and ATU installation) in conjunction with the WWTP upgrade still provided a substantial reduction in nutrients and potential impacts to marine habitat quality relative to the present situation at a price point roughly $100 million less than the entirely ATU option. Of note, results were more sensitive to the inclusion of the WWTP upgrade option than they were to assumptions regarding the efficiency of the cesspool conversion technologies. The model outputs also suggest that the spatial distribution of potential impacts should be carefully considered when comparing different wastewater management scenarios. When evaluated separately, the WWTP option reduced total nutrients by more than the targeted cesspool conversion option at a fraction of the cost. However, potential improvements in marine habitat quality only occurred in the immediate vicinity of the WWTP, whereas the benefits under targeted cesspool conversion were more evenly distributed along the coast.

Priority watershed management areas for groundwater recharge and drinking water protection: A case study from Hawai'i Island.

Worldwide, water utilities and other water users increasingly seek to finance watershed protection and restoration in order to maintain or enhance water quality and quantity important for drinking water supply and other human use. Hydrologic studies which characterize the relative effectiveness of watershed management activities in terms of metrics important to water users are greatly needed to guide prioritization. To address this need, we worked with a local water utility in Hawai'i to develop a novel framework for prioritizing investments in native forest protection and restoration for groundwater recharge and applied it in the utility's priority aquifers and recharge areas. Specifically we combined land cover and water balance modeling to quantify the 50-year cumulative recharge benefits of: 1) protection of native forest from conversion to non-native forest, and 2) restoration of native forest in non-native grasslands. The highest priority areas (80th percentile of benefits) for native forest protection are projected to prevent the loss of over 48,600 m3 per hectare of recharge over 50 years. Incorporating land cover change modeling (versus assuming all areas are equally susceptible to invasion) shifts prioritization towards low to mid-elevation mesic forest areas at the highest risk of invasion by invasive canopy species as well as to high elevation, cloud forest areas at high risk of conversion to non-native grassland or bare ground. We also find that, in the highest priority areas with substantial fog interception, native forest restoration is projected to increase recharge by over 88,900 m3 per hectare over 50 years, but that decreases in recharge occur in areas with low fog interception. This study provides a framework for prioritizing investments in forest protection and restoration for groundwater recharge in a way that incorporates both the threat of conversion as well as changes in hydrologic fluxes. The framework and results can be utilized by current managers and updated as new ecohydrological data become available. The results also provide broad insights on the links between watershed management and groundwater recharge, particularly on islands and in other regions where species invasions threaten source watersheds and where groundwater is a primary water source.

Collaborative research to support urban agriculture in the face of change: The case of the Sumida watercress farm on O'ahu.

As urban areas expand around the world, there are growing efforts to restore and protect natural and agricultural systems for the multitude of ecosystem services they provide to urban communities. This study presents a researcher-farmer collaboration in a highly urbanized area of O'ahu focused on understanding the historical and current challenges and opportunities faced by a culturally and socially valued spring-dependent urban farm, Sumida Farm, which produces the majority of the state of Hawai'i's watercress. We conducted a long-term trend analysis (25 years) of factors identified by the farmers to be important historical drivers of crop yield, including groundwater pumping, pest outbreaks, temperature, Oceanic Niño Index, and precipitation. We combined this analysis with a year of intensive spring water sampling on the farm to evaluate nutrient and contaminant composition and flow to understand water-related stressors, as well as evaluate the potential of the farm to provide nutrient retention services. We found negative correlations between historical crop yields and increases in the Oceanic Niño Index, temperature thresholds, and pest outbreaks. Despite the surrounding urbanization, we found on-farm water quality to be very high, and microbial analyses revealed an abundance of denitrifiers (nirS gene) suggesting that the farm provides a nutrient retention service to downstream systems. Finally, we found that socio-cultural values including heritage value, aesthetic value, and educational value are increasingly important for the Sumida family and surrounding community. These socio-cultural benefits alongside highly valued local food production and nutrient retention services are essential for continued community and political support. Collectively, our study demonstrates that challenges facing urban agricultural systems shift through time, and that recognition of the beyond crop-yield benefits of these systems to urban communities is essential to their long-term survival.

Optimal multi-instrument management of interrelated resources and a groundwater dependent ecosystem.

We develop and operationalize an integrated groundwater and watershed management model using data from the Kiholo aquifer on the west coast of Hawai'i Island. Results from a numerical simulation suggest that investment in fencing (passive management) is preferred to invasive species removal (active management) if we are limited to selecting a single conservation tool. However, using both instruments jointly increases net present value relative to using either instrument independently in most cases tested, and the additional benefit of invasive species removal increases as water becomes scarcer. The general results are largely insensitive to variations in the invasive species uptake rate and recharge benefits of fencing, and in all cases, use of both instruments reduces the loss resulting from the imposition of a safe minimum standard for groundwater-dependent ecosystems more effectively than either instrument alone.

Contributions of native forest protection to local water supplies in East Maui.

Tropical forests provide a suite of benefits including biodiversity, cultural value, and a range of ecosystem services. Globally, there is increasing interest in incentivizing native forest protection as a multi-benefit natural infrastructure strategy to secure clean and ample water supplies. In addition to conversion to agriculture and other non-forest land uses, non-native species invasion represents a major threat to these systems, particularly on islands. Whereas several recent efforts have quantified the benefits of reforestation or avoided agricultural expansion in tropical forest areas, the hydrologic and associated economic benefits of avoided invasion have received less attention. To address this gap, we quantified the benefits of protecting native forest from conversion to non-native forest in East Maui, Hawai'i in terms of groundwater recharge, a highly valued hydrologic ecosystem service that water utilities increasingly seek to co-finance. Compared with two counterfactual invasion scenarios, the groundwater recharge benefits of planned conservation activities reached 40.9 to 146.3 million cubic meters over 100 years depending on invasion rate assumptions. This translated to 2.70 to 137.6 million dollars of cost savings to the water utility in present value terms (achieved through reducing reliance on more expensive water alternatives) under a range of discount rates and water scarcity assumptions. Our results suggest that investing in native forest conservation provides an important hydrologic ecosystem service benefit that complements the range of benefits provided by these ecosystems. These findings demonstrate that co-financing native forest conservation represents an important supply side option in water resources planning.