Impact to Stream Water Quality from Sewage Exfiltration and Legacy On-Site Disposal Systems on the Island of O‘Ahu, Hawaii

Hawai`i was the last state in the United States to ban the construction of new cesspools.  The construction of new cesspools was banned in 2015 due to concerns about water quality threats to human and coral reef health. Legislation was passed in 2017 to replace all existing cesspools in the state by 2050.

A survey completed in 2009 found that 14,600 On-Site Disposal Systems (OSDS) exist on the most densely populated Hawaiian Island, O‘ahu, mostly within one-kilometer of the coast and in residential neighborhoods [1]. These OSDS include cesspools, which account for 77 percent of the surveyed total, along with septic tanks and aerobic treatment units. Cesspools provide minimal treatment of wastewater before it percolates to the underlying groundwater aquifer. This survey estimated that 38 million liters per day (mld) of wastewater are released to the environment island-wide from OSDS, with the majority of the released sewage reaching underlying groundwater. High densities of OSDS are found throughout O‘ahu. Estimates of OSDS density and daily wastewater release volumes range from 60 units/kilometer²(km²) and 1.5 mld in leeward urban O‘ahu (Makiki), 27 units/km² and 2.3 mld in windward O‘ahu (Waimanalo) and 20 units/km² and 3.3 mld on the north shore of O‘ahu (Waialua). The authors of the OSDS survey asserted that the sheer numbers of OSDS in some communities on O‘ahu produces a cumulative effluent volume that is comparable to that of municipal wastewater treatment plants (WWTP) [1].

Groundwater quality on O‘ahu is also impacted by exfiltration of sewage from the island’s 3,400-kilometer web of sewer lines that convey an average of 394 mld sewage to WWTP operated by the City and County of Honolulu. The age range distribution of the sewage conveyance system is as follows: <25 years; 22.2%, 26-50 years; 18.3%; 51-75 years; 42.7%, 76-100 years; 11.8% and >100 years, 1.2%. In the urban areas surrounding Honolulu, the majority of the sewage conveyance system is over 65 years old and is beset by leaking joints and pipes due to the presence of corrosive hydrogen sulphide gasses produced by the combination of a warm tropical climate and the high sulfate content of the shallow groundwater surrounding the pipes and joints in the coastal portions of the conveyance systems. Maintenance of this aging and deteriorating sewage conveyance system is a challenge, and there is an ongoing need to replace and upgrade Hawaii’s sewer lines and force mains due to both capacity and structural integrity issues [2].

The United States Environmental Protection Agency (USEPA) reported between 12 and 49% of wastewater flows are lost due to leaking infrastructure in United States cities [3]. The system-wide amount of wastewater exfiltration from O‘ahu’s sewage system is unknown. A rough estimate of 10% system-wide exfiltration is based upon the difference in average monthly groundwater withdrawals by the water utility and the average monthly volume of wastewater treated at the WWTPs operated by the City and County of Honolulu [4]. A 10% loss suggests around 40 mld of wastewater is released to the environment from exfiltration from the sewage conveyance system, roughly the same amount of wastewater that is estimated to be released from the ongoing use of OSDS. Evidence of exfiltration from the sewage conveyance system has been observed in two coastal areas in urban Honolulu, Waikiki and Mapunapuna, based on the presence of pharmaceutical compounds in storm drains in these areas and in the Ala Wai Canal, which receives input from Waikiki storm drains [4].

The United States Geological Survey (USGS) estimates that the mean island-wide groundwater flux entering the ocean is around 2,050 mld [5]. The combined estimated island-wide release of wastewater due to the continued use of legacy OSDS and from exfiltration from the sewage conveyance system is around 80 mld, or roughly 3.9% of the total island-wide groundwater flux to the ocean. The simulated groundwater flux along O‘ahu’s shoreline varies from less than 4 mld/km in the drier and narrower portions of the island to maximum values of around 76 mld/km along the shoreline bordering Pearl Harbor [5]. based upon these previous estimates of wastewater input, chemical evidence of wastewater should be readily detected in groundwater that underlies and streams that flow through areas on O‘ahu with high OSDS and sewer line densities. The focus of the paper is to evaluate whether the combined environmental impact from legacy cesspools and sewage exfiltration can be estimated by measuring the pharmaceutical and nutrient concentrations in island streams and springs under baseflow conditions, when the water present in the streams almost exclusively originated from groundwater discharge from the surrounding watershed. Sampling was also conducted under low water-level conditions in Lake Wilson, which is the only location on O‘ahu that receives direct wastewater input from a WWTP servicing Central O‘ahu.

1.1 Characteristics of Hawaiian streamflow

Streams on O‘ahu are typically short (<6 km long) and have relatively steep gradients [6]. Rainfall runoff to O‘ahu streams tends to be flashy due to the high intensity of rainfall that often falls in the island’s interior combined with the small size of the steep drainage basins and limited channel storage present within the island’s watersheds. The permeable nature of the island’s volcanics and upland soils allow rapid infiltration of water to underlying aquifers resulting in very few streams on O‘ahu being perennial over their entire reach. The two largest perennial streams on O‘ahu, Kahana and Waikele streams, have annual median streamflow of around 1.05 cubic meters per second (cms). The majority of perennial streams on O‘ahu have annual daily median flows of less than 0.1 cms. The flashy nature of runoff and the paucity of large perennial streams precludes large-scale development of surface-water on O‘ahu and leads to near-total reliance on groundwater for water supply for the island’s roughly 1 million residents [6].

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Figure 1. Increase in stream specific conductance during flow through urban O‘ahu

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Figure 2. Variation in specific conductance in Kawa stream in response to rainfall

Perennial streamflow occurs on O‘ahu in watersheds that receive significant groundwater discharge. These include areas where erosion has incised valleys into rift zones and the stream course intersects the elevated water tables created by the nearly impermeable dikes within the rift zones of the volcano, in upland areas, and near sea level where the stream course intercepts shallow perched or basal groundwater.

Groundwater discharge to streams is known as baseflow, which becomes the dominant component of streamflow during periods of dry weather. Dike impounded groundwater is the dominant contributor to baseflow in streams on O‘ahu. Perched groundwater also contributes to stream baseflow, especially in alluvium and rocks associated with the post-erosional phase of activity encountered in valleys in the Honolulu area and in windward O‘ahu. basal groundwater can also enter the coastal sections of streams where the bottom of the stream intercepts the top of the basal lens between streambed elevations of 0 to 7.6 meters above sea level in areas where no coastal confining layer exists. Groundwater can also enter streams through springs. Springs form when a geologic structure (e.g., fault or fracture) or topographic feature (e.g., side of a hill or a valley) intersects groundwater either at or below the water table. This can discharge groundwater onto the land surface, directly into the stream, or into the ocean [7]. Under drought conditions, streamflow in windward O‘ahu streams, where flow is dominated by dike impounded groundwater discharge, can decrease to about half the median flow, while flow in perennial streams in leeward O‘ahu can decline to between 10 and 25% the stream’s median flow [8].

Streams in Hawai`i gain water along some reaches and lose water along other reaches depending on local geohydrologic conditions [7]. A gaining stream is simply one in which groundwater flows into the stream and adds to the discharge of the stream.

During periods of low flow, the quality of the groundwater controls surface-water quality. A vertical profile of samples was collected from two deep monitoring wells located in Central O‘ahu and analyzed for solutes and isotopes [9]. The upper 100 meters of the basal aquifer in Central O‘ahu was composed of water recharged from local rainfall and irrigation return water that fell or was applied over the previous few decades. Wastewater released due to the continued use of OSDS and from sewage exfiltration would also impact the upper portion of the underlying perched or basal aquifers encountered as wastewater percolates through the vadose zone. This upper portion of the basal and perched groundwater bodies is the zone that contributes groundwater to streams in their lower reaches where samples were collected during this study.

Figure 1 shows the increase in specific conductance measured in four streams in leeward O‘ahu as they flow from the mountains toward the sea. This increase in specific conductance reflects the higher concentration of dissolved chemicals (anions, cations, nutrients, pharmaceuticals, etc.) present in groundwater that discharge to these streams at lower elevations compared to the levels present from water discharging from the dike impounded rift zones, which serves as the source of baseflow in the upper reaches of the streams.

Figure 2 shows the variation in specific conductance measured in Kawa stream as a result of rainfall over an eight-day period in 2021. Kawa stream is a small (0.08 cms average annual flow), perennial spring fed stream in windward O‘ahu. The upper monitoring site was located just downstream of the upper springs which supply baseflow to the stream, while the lower monitoring site was located one-kilometer downstream. Flow in this stream becomes dominated by groundwater input within a day or two after moderate rainfall events based on the rapid recovery in specific conductance to spring conductance values (~300 µS/cm).

1.2 Anthropogenic markers of sewage contamination-carbamazepine, sulfamethoxazole and caffeine

Carbamazepine, sulfamethoxazole and caffeine are known as emerging contaminants, which describes pollutants that have been detected in water bodies, may have ecological or human impact, and typically are not regulated under current environmental laws. These compounds are also known as micropollutants because they are typically present in trace quantities (part per trillion to billion levels) in the environment. These micropollutants enter the environment during our daily routines as we consume, flush away, or wash these compounds down the sink. Carbamazepine, caffeine and sulfamethoxazole represent the most frequently detected pharmaceuticals in treated wastewater [10]. As a result, these compounds are increasingly used as anthropogenic markers of sewage contamination.

Carbamazepine is a commonly prescribed anticonvulsant, pain relief and bipolar disorder treatment drug. Sulfamethoxazole is an antibiotic prescribed to treat common bacterial infections including urinary tract infections, middle ear infections, bronchitis and diarrhea. Caffeine is a naturally occurring stimulant found in coffee, soda, tea, chocolate and energy drinks. Carbamazepine, caffeine and sulfamethoxazole were the first, third and sixth (62.3%, 56.1%, 40.7%) most commonly detected pharmaceutical compounds in 1,052 river samples collected from 104 countries worldwide in a recently published global-scale study [11]. Caffeine was detected in river samples collected from every continent while carbamazepine was detected in rivers on all continents except Antarctica and sulfamethoxazole on all continents except Antarctica and Oceania (Australia and New Zealand). The common detection of carbamazepine and sulfamethoxazole is attributed to their conservative behavior in the environment with carbamazepine being somewhat more persistent. The common detection of caffeine is due the high levels and ubiquitous nature of its consumption. The average concentrations of carbamazepine, sulfamethoxazole and caffeine measured in streams worldwide were 85, 262 and 1,510 nanograms per liter (ng/L), respectively [11].

The relative stability of carbamazepine and sulfamethoxazole enables detection at larger distances from sewage release sites while caffeine serves as a better tracer for detecting recent, proximal releases of sewage or grey water, given its ephemeral nature and higher initial concentration levels in released untreated wastewater.

1.3 Pharmaceutical concentrations in sewage in Hawaii

Researchers at the University of Hawaii measured the concentration of caffeine in sewage in the main sewer line exiting Manoa Valley on O‘ahu to range from 5,000 to 103,400 parts per trillion (ppt, ng/L), with the highest flux of caffeine (2.4 milligrams/second) exiting the valley between 8 and 11 AM [12]. The Safe Drinking Water branch of the State of Hawaii Department of Health (HDOH) conducted two rounds of sampling of raw wastewater influent at four WWTP and also collected 13 samples of treated wastewater effluent generated at 13 WWTP facilities that produce reclaimed water throughout the State of Hawai‘i [13]. LC-MS-MS methods were used to analyze these samples. Carbamazepine was not detected in the wastewater influent; likely due it being masked by other constituents present in the wastewater during analysis.

Table 1. Carbamazepine, Sulfamethoxazole and Caffeine concentrations in Septic Tanks and WWTP Effluent and Influent in Hawai‘i

During the current study, samples were collected from septic tanks at various beach parks on the island of O‘ahu and analyzed for carbamazepine (26 samples), caffeine (20 samples) and sulfamethoxazole (10 samples) using an Enzyme Linked Immunosorbent Assay (ELISA) immunoassay method. Table 1 summarizes the carbamazepine, sulfamethoxazole and caffeine concentrations measured in the septic tanks along with levels measured by HDOH in influent and effluent to Hawaii WWTPs. Using the average concentrations of carbamazepine measured in septic tanks and the average of the sulfamethoxazole and caffeine concentrations measured in the septic tanks and in WWTP influent, the mean concentration of carbamazepine, sulfamethoxazole and caffeine in Hawaii wastewater is around 745, 1,300 and 61,000 ppt, respectively. The mean concentration of ammonia, nitrate (NO3-N) and total phosphate measured during this study in wastewater collected from septic tanks (eight samples) located at a busy public beach park used by a representative cross section of island residents and tourists was 80, 13 and 47 milligrams per liter (mg/l), respectively.

1.4 Pharmaceutical levels measured in coastal urban settings of Honolulu

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Figure 3. Contoured maximum carbamazepine concentrations in storm drains in the tourist and residential Waikiki and McCully/Moiliili areas

Thirty six streams and 11 springs located down-gradient of areas with varying densities of OSDS and sewage lines on the island of O‘ahu were sampled over a two-year period. A minimum of four samples were analyzed for pharmaceuticals and nutrients from all streams and springs. Caffeine was present in 80% and 94% of the streams and springs sampled above detected (175 ppt) and estimated (80 ppt) concentration levels, respectively. Sulfamethoxazole was present in 6%, 15% and 15% of the streams and springs sampled above detected (25 ppt), estimated (12 ppt) and trace (>1 ppt) concentration levels, respectively. Carbamazepine was present in 3%, 5% and 42% of the streams and springs sampled above detected (25 ppt), estimated (12 ppt) and trace (>1 ppt) concentration levels, respectively. Trace levels of caffeine (>80 ppt), carbamazepine and sulfamethoxazole (12 ppt) were measured in 70%, 23% and 6% of the samples collected, respectively.

Table 2 summarizes the average pharmaceutical and nutrient concentrations measured in streams and springs located in the following four categories of contributory land use: (1) areas with high densities of cesspools and high densities of sewer lines; (2) areas with high densities of cesspools and low densities of sewer lines; (3) areas with low densities of cesspools and high densities of sewer lines; and (4) areas with low densities of cesspools and low densities of sewer lines. Table 2 also includes the pharmaceutical and nutrient data collected from Lake Wilson and its two outlets (Kaukonahua Stream and Wahiawa Ditch).

Table 2. Pharmaceutical and nutrient concentrations in O‘ahu Streams, Springs and Lake Wilson

Figure 6 shows the average caffeine concentrations measured in the streams and springs sampled during this study in relation to areas of high OSDS and sewer line densities.

Three springs (Waimanalo seep, Ulupo Heiau and Waimano spring) contained trace or estimated concentration levels of carbamazepine during the two rounds of reconnaissance sampling conducted in November 2020. The Waimanalo seep had the most consistently detectable concentrations levels of carbamazepine and caffeine of the three springs repetitively sampled. Figure 7 shows the persistent nature of and variability in concentration levels of carbamazepine and caffeine in the Waimanalo Seep over the 23-month period sampled. Waimanalo Seep is located in a narrow portion of the island with relatively low coastal groundwater flux rates.

Nitrate, ammonia and phosphorus can be indicators of the presence of wastewater since these nutrients are enriched in wastewater. The average concentration of nitrate, ammonia and total phosphate measured in wastewater collected from septic tanks was 13, 80 and 47 mg/l, respectively. However, nitrate is not unique to wastewater and on O‘ahu its presence in groundwater fed streams may reflect historical use of fertilizers, especially in areas formerly used for sugarcane cultivation [14]. High concentrations of nitrate (30 to 80 mg/kg) are adsorbed on positively charged iron and aluminum oxide particles in the subsoil and saprolite beneath pineapple and sugarcane cultivation areas of central O‘ahu at depths between 6 to 20 meters [15]. Infiltration percolating through the soils and saprolite underlying lands formerly used for sugar cultivation provides a continuous input of nitrate to the underlying basal groundwater aquifers.

The historic use of phosphorus fertilizers has not significantly affected groundwater quality on O‘ahu because phosphate is retained in the aluminum and iron minerals in the soils [7]. Most of the silica in stream water is derived from leaching of volcanic rocks and soil. Rainfall has silica concentrations of less than 1 mg/l and infiltrating water will dissolve the silica present in volcanic rocks and soils leading to silica concentrations in the basal aquifer of between 15 to 40 mg/l [16]. Groundwater concentration levels of above 40 mg/l silica are associated with areas impacted by return irrigation water [14].

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Figure 6. Average caffeine concentrations measured in streams and springs

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Figure 7. Carbamazepine and caffeine concentrations in Waimanalo Seep

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Figure 8. Nitrate concentrations measured in O‘ahu streams and springs

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Figure 9. Silica concentrations measured in O‘ahu streams and springs