Drastic reduction in carbon dioxide (CO₂) emissions supplemented with active removal of 100–1,000 Gt CO₂ from the atmosphere over the 21st century is necessary to limit global warming to below 2°C (UN, 2015; IPCC, 2018; Rogelj et al., 2018; IPCC, 2023). Among potential carbon dioxide removal (CDR) methods, marine geochemical approaches to capturing and storing CO₂ in seawater are gaining attention for their potential durability and ability to scale (Rau, 2011; National Academies of Sciences and Medicine, 2022; Cross et al., 2023). One promising approach to mCDR is ocean alkalinity enhancement (OAE) (Renforth and Henderson, 2017; Cai and Jiao, 2022; Oschlies et al., 2023; Cong et al., 2024). OAE is considered a promising potential approach for mCDR given its relatively low potential cost per ton of CO₂ removed from the atmosphere and potential to reach Gt-scale removal compared to other more expensive technologies like direct air capture (Cross et al., 2023). By elevating the alkalinity of seawater, OAE alters the speciation of dissolved inorganic carbon, and encourages the net transfer of CO₂ into the ocean either through an increase in net ocean CO₂ uptake or a reduction in net outgassing (Renforth and Henderson, 2017; Cai and Jiao, 2022; Schulz et al., 2023; Cong et al., 2024). The shifts lead to a durable (i.e., long-term) enhanced storage of ocean carbon relative to expectations without the OAE intervention, predominantly as elevated dissolved bicarbonate concentrations (Renforth and Henderson, 2017; Cai and Jiao, 2022; Schulz et al., 2023; Cong et al., 2024; Khangaonkar et al., 2024).

Multiple approaches to elevating seawater alkalinity have been proposed, including the addition of alkaline materials to seawater and the electrochemical conversion of seawater or brine into alkaline solutions that are delivered to the surface ocean (Cai and Jiao, 2022; Eisaman et al., 2023; Oschlies et al., 2023; Cong et al., 2024). The type of alkalinity and method of delivery impact the resulting chemical changes in the receiving seawater, but, in general, we expect an increase in pH and a rapid shift of dissolved inorganic carbon (DIC) speciation toward carbonate (CO₃ ^2-). This increase in pH, in turn, will also reduce the concentration of dissolved CO₂ gas in seawater (pCO_2(sw)) (Montserrat et al., 2017; Cai and Jiao, 2022; Eisaman et al., 2023; Hartmann et al., 2023; Schulz et al., 2023; Cong et al., 2024; Suitner et al., 2024). These changes are greatest near the point of alkalinity-enhanced seawater discharge and are diluted until the signal is indistinguishable from baseline variations (Khangaonkar et al., 2024; Hashim et al., 2025). Over weeks to years, pCO_2(sw) re-equilibrates—or at least returns to values observed prior to alkalinity enhancement in regions where natural processes result in persistent disequilibria—due to changes in the net exchange of CO₂ across the air-sea boundary. Under typical ocean conditions, OAE results in an increase of 0.7–0.9 mol of DIC per mole of alkalinity added to seawater, though this value depends on the efficiency of alkaline delivery and air-sea equilibration. That is, the increases may be less when alkaline feedstocks do not fully dissolve or are added in regions where, for example, density currents drive alkalinity-enhanced seawater out of contact with the atmosphere (Fennel et al., 2023; He and Tyka, 2023; Ho et al., 2023; Wang et al., 2023; Khangaonkar et al., 2024; Ringham et al., 2024; Yankovsky et al., 2024; Zhou et al., 2024; Anderson et al., 2025). However, even if the alkalinity-enhanced seawater is out of contact with the atmosphere, it may still be beneficial for ameliorating ocean acidification in subsurface waters where applicable (Renforth and Henderson, 2017; Cai and Jiao, 2022; Oschlies et al., 2023; Bednaršek et al., 2025).

OAE is promising due to its potential scalability, the high durability of OAE CO₂ storage, and potential co-benefits, including the potential for local mitigation of seawater acidification (Renforth and Henderson, 2017; Cai and Jiao, 2022; Oschlies et al., 2023; Bednaršek et al., 2025). For these reasons, and in response to the urgency of developing climate solutions, efforts are simultaneously underway to research, test, and commercialize OAE technologies (National Academies of Sciences and Medicine, 2022; Palter et al., 2023; Carbon to Sea Intiative, 2024; Cross et al., 2024; Carbon to Sea Intiative, 2025; ICES, 2025; Travis, 2025; Yao et al., 2025). This study describes the deployment of a demonstration electrochemical OAE system as part of a collaboration between private (Ebb Carbon, Inc.), academic (University of Washington), and federal (Pacific Northwest National Laboratory; NOAA Pacific Marine Environmental Laboratory) partners, resulting in the dispersal of electrochemically generated aqueous alkalinity through a wastewater treatment plant (WWTP) at the Pacific Northwest National Laboratory (PNNL) into Sequim Bay, Washington, United States. The research conducted in this study was part of the NOAA-funded project EASE-OA (Electrochemical Acid Sequestration to Ease Ocean Acidification), which has included multiple components to advance towards OAE field trials, including: 1) testing and development of Ebb Carbon’s electrochemical system; 2) simulation of continuous aqueous alkalinity-enhanced seawater discharge into Sequim Bay (Khangaonkar et al., 2024); 3) evaluation of the impact of aqueous alkalinity on local ecologically important eelgrass epifauna (Jones et al., 2024); and 4) monitoring CO₂ uptake in mesocosm-scale ex-situ experiments of seawater treated with aqueous alkalinity.

Ebb Carbon’s approach to OAE is to utilize bipolar membrane electrodialysis (BPMED) to generate aqueous alkalinity from seawater that then returns to the surface ocean, while simultaneously removing acid from seawater (Eisaman et al., 2012; Eisaman, 2024). The minimum potential for reaction of water dissociation through BPMED compared to electrolysis is lower, 0.83 V versus 2.06 V (Eisaman et al., 2012; Eisaman et al., 2023; Kulkarni et al., 2024). Therefore, BPMED is a lower energy method for alkalinity production and in turn less expensive, which makes it an economically appealing option for OAE based CO₂ removal (Eisaman et al., 2012; Eisaman et al., 2023; Cross et al., 2024). The Ebb Carbon BPMED system in this study, designed to remove 100 t of atmospheric CO₂ yr^-1, began operating at PNNL-Sequim in August 2023. This system processes seawater sourced from Sequim Bay through the PNNL-Sequim facility to generate hydrochloric acid (HCl), sodium hydroxide (NaOH), and brine streams (Eisaman et al., 2012; Eisaman et al., 2023; Eisaman, 2024). The HCl produced is 0.5–1.0 mol L^-1 and is utilized for PNNL-Sequim research focused on developing novel uses for the acid byproduct. While research is an efficient use of acid byproduct for a pilot scale BPMED implementation, the use for the larger quantities of HCl byproduct generated by scaled electrochemical OAE deployments is an active area of research. Potential use cases include algal cultivation, mineral extraction, and industrial applications (Eisaman et al., 2023; Eisaman, 2024; Hibbeln et al., 2024). The NaOH is mixed with the brine stream and remaining seawater for release back to the surface ocean. The enhanced alkalinity stream resulting from the Ebb Carbon system at PNNL-Sequim can vary in concentration from 0.2–1.0 mol L^-1 NaOH and a practical salinity between 20–70. NaOH-based OAE research has been conducted in multiple laboratories (e.g., Hartmann et al., 2023; Iglesias-Rodríguez et al., 2023; Bach et al., 2024; Ferderer et al., 2024; Ringham et al., 2024; Britton et al., 2025), and a limited number of completed or planned field studies (i.e., Albright et al., 2016; Subhas et al., 2024), typically using the addition of 0.5–1 mol L^-1 NaOH to raise seawater alkalinity.

While there is attention on the release of NaOH from ships for OAE research [i.e., LOC-NESS (Permit #EPA-HQ-MPRSA-2024–002)], the primary delivery method for electrochemically generated aqueous alkalinity in commercial projects is point source dispersal through a coastal outfall (Cai and Jiao, 2022; Cong et al., 2024). Within the US, coastal outfalls are regulated under the Clean Water Act, which requires National Pollutant Discharge Elimination System (NPDES) permits. Permits are tailored to specific provisions to ensure that any given pollutant discharge does not harm water quality or human health. Coastal outfalls are typically designed such that a released effluent quickly mixes into the natural environment. The initial discharge may have requirements for specific conditions (such as a maximum pH or concentration limit) by the ‘end-of-pipe’ before discharge into the natural environment or may include a mixing zone. A mixing zone is a regulatory concept used to specify a volume of receiving water centered on an outfall in which water quality standards may be temporarily exceeded, but by the edge of which the diluted effluent is indistinguishable from natural ambient conditions (EPA 820-B-14–004). Release of aqueous alkalinity through existing NPDES-permitted coastal outfalls and within the specified permit limits may be sufficient for compliance with existing standards, allowing for some OAE research. PNNL-Sequim has an onsite WWTP that is permitted for the release of wastewater within pH_NBS 6.0–9.0 at ‘end-of-pipe’. The permit allows WWTP discharge of seawater that is either treated with only enough alkalinity to remain within this pH range or with additional alkalinity and then allowed to re-equilibrate with atmospheric CO₂ and return to this range. As of this study, only one NPDES permit has been granted for an mCDR-specific process, specifically that of Ebb Carbon’s nearby Project Macoma pilot in Port Angeles, WA, which allows for routine discharge of alkalinity-enhanced seawater below pH_NBS 9.8 into a defined mixing zone (Permit #WA0991051). There are advantages to constraining alkalinity additions below pH_NBS 9.0, namely, adaptation to existing coastal outfalls permitted under this condition and limitations of concerns over environmental impacts and secondary precipitation. However, a lower pH maximum creates challenges for the release of aqueous alkalinity at increased scales, such as additional energy required to pump the seawater needed for dilution of the pH perturbation to the lower value or increased equipment and complexity required to pre-equilibrate alkalinity-enhanced seawater before release.

We released electrochemically generated aqueous alkalinity through the onsite WWTP at PNNL-Sequim into Sequim Bay, WA, in two experiments conducted in November 2024 and February 2025. Our first objective was to determine our ability to maintain, monitor, and deliver alkalinity-enhanced seawater through the multiple steps of the facility out to the outfall, and this objective was achieved. The second objective was to test our ability to detect the increased alkalinity from the outfall above ambient conditions, despite the upper pH_NBS limit of 9.0 and the presence of high tidal currents in this study area. Monitoring OAE in tidally energetic regions can be difficult due to rapid mixing and dilution effects, as well as large natural variability in carbonate chemistry due to these tidal fluctuations (Khangaonkar et al., 2024). Herein, we describe: 1) experimental design and release of alkalinity through the WWTP; 2) monitoring capabilities throughout the facility and extending beyond the coastal outfall; 3) near-field modeling results to estimate the dilution of the pH signal from the outfall. This study represents the first proof-of-concept of using existing coastal outfalls for electrochemical OAE research.

As a first trial, the November 2024 alkalinity-enhanced seawater release was used to test our ability to monitor and maintain an elevated pH_NBS ≤ 9.0 while remaining within compliance of water quality standards at the outfall. The initial temperature, salinity, and pH_NBS of seawater at the beginning of the alkaline release ranged from 9.8°C–9.9°C, 31.9–32.0, and 7.5–7.6, respectively (Table 1). pH_NBS was increased to 9.0 from ambient conditions of 7.5–7.6 within the mixing tank (Supplementary Figures S2, S5). The pCO₂ in the mixing tank decreased from ambient conditions of 772–796 μatm to 70–110 µatm in November, as measured by the ASVCO₂ system (Table 1). The alkalinity enhanced seawater was pumped into GW Cell 3, where pH_NBS was observed at 8.9, with the slight reduction likely due to mixing with residual water remaining within the cell (Supplementary Figures S3, S5). pH_NBS again decreased within GW Cell 2 to 8.7, which was released from the WWTP outfall (Supplementary Figures S4, S5). The pH_NBS at the outfall during and after the alkaline release did not deviate from ambient conditions (Supplementary Figures S5, S5), indicating that sensors deployed on the outfall pipe were either not located in line with the alkalinity-enhanced seawater discharged or the pH_NBS signal was diluted too rapidly to be captured at this point. No alkaline signal was determined at the pier, however there was a flood tide during this time that could have diverted water into the Bay away from the pier rather than toward the inlet (Supplementary Figures S6, S5). A comparison of results from the YSI sondes and pH calculated from DIC/TA samples (on total and NBS scale) within the mixing tank and GW cells indicated that the sondes slightly underestimated pH (Table 1). However, the final release of alkaline seawater at the outfall remained at pH_NBS < 9.0 and within regulatory monitoring requirements, even if we adjust for the apparent measurement bias. While pH calculated from DIC and TA is potentially more accurate than measuring with a sonde, this is not practical from a compliance monitoring standpoint, given that the analysis of samples is often delayed resulting in limited usefulness in real-time decision making.

The second alkalinity-enhanced seawater release in February 2025 (∼180,300 L) was ∼6.9x larger than the volume released in November 2024 (∼26,200 L) and was discharged into Sequim Bay during four separate pulses through the WWTP (Figure 3). The initial temperature, salinity, and pH of seawater at the beginning of the alkaline release ranged from 7.3°C–7.9°C, 31.3–31.5, and 7.6–7.7, respectively (Table 2). pH_NBS was increased to 8.9–9.1 from ambient conditions of 7.6–7.7 within the mixing tank (Supplementary Figures S7, S6). For the February trial, seawater pCO₂ data was not recorded by the ASVCO₂ system in the mixing tank prior to base addition. Therefore, the ambient conditions were estimated using the range in pCO₂ from the SAMI-CO₂ at the outfall within 12 h prior to the start of the experiment. Based on the SAMI-CO₂ and ASVCO₂ data, the baseline pCO₂ was between ∼812 and 846 μatm, which decreased to 35–52 µatm (Table 2; Supplementary Figure S10). The pH_NBS was maintained through GW Cells 3 and 2 at 9.0 and 8.5–9.0, respectively, across all four pulses of alkalinity-enhanced seawater discharge (Supplementary Figures S6, S8, S9). The decrease at the end of the final pulse is likely due to GW Cell 2 no longer receiving alkalinity-enhanced water from GW Cell 3. Again, pH measured via YSI sondes was lower than that calculated via CO2SYS from DIC/TA samples in the mixing tank and GW cells (Table 2). However, the final release of alkaline seawater at the outfall remained <9.0, within regulatory monitoring requirements (Supplementary Figures S10, S6). In contrast to the November trial, there was a detectable increase in pH_NBS at the outfall over the four pulses during the February trial. During all four pulses, the Bay was in high tide conditions with a slight increase in water surface elevation over the day (Supplementary Figure S1).

Temperature, salinity, and pH_NBS were measured continuously for a week at the outfall and pier, beginning 3 days before the alkalinity-enhanced seawater release and continuing for 4 days afterwards (Figure 7). The sensors at the outfall were positioned closer to the center line of flow from the outfall during the February release, which improved monitoring capabilities and allowed for the detection of the four pulses of increased pH (reaching a peak pH_NBS 8.3) (Figure 4). Ambient seawater pH_NBS ranged from 7.6–7.7. The YSI Exo 2 sonde recorded a higher peak pH_NBS (8.3) than that of the SAMI-pH (8.1), which only captured an increase in pH during the first and fourth pulsed releases from the WWTP due to the reduced measurement frequency (15 min) and perhaps slightly different positioning relative to the outfall (Figures 6, 7). The YSI Exo2 sonde was in the direct line of the outfall plume compared to the SAMI, which is slightly higher in the water column (Figure 4). In addition, current speeds were faster during the second release than the first, which may have diluted the pH signal more quickly and made it more difficult to capture (Supplementary Figure S12). The pH rapidly returned to ambient values between pulses, ranging from 7.4–7.7 via YSI sonde or 7.7–7.9 via SAMI-pH (Figures 6, 7). No shift in pH was measured by either the SAMI-pH or Seabird Hydrocat EP-V2 at the nearby pier during or after the alkaline releases (Supplementary Figures S7, S6, S11). SAMI-pH and Seabird Hydrocat data had strong agreement in pH_NBS measurements with one another as well as with the discrete samples collected off the pier (Table 2; Figures 6, 7). The higher accuracy of both the SAMI-pH and discrete sample data relative to the Seabird Hydrocat or YSI sonde, along with agreement between the pier sensor and sample data, indicate that the pH measurements made with the YSI sondes are biased low by 0.1–0.3 pH (Table 2). The high frequency variability in pH during the alkalinity-enhanced seawater discharges demonstrates the need for frequent sampling or continuous records, while the differing sensitivity of the SAMI-pH and YSI sonde pH records to the discharges likely reflects the strong heterogeneity of the impacts of any point source release into an energetic mixing environment. The differing accuracy and performance of these sensors versus sampling resolution demonstrates that the choice of sensor in a field experiment can matter greatly.

The treatment process of the seawater to enhance alkalinity did not appear to affect the temperature for the February release (Table 2). While higher temperatures were observed during the release period, they were not outside of those observed naturally in the system (Figure 7). There was also no measured increase in salinity associated with the alkalinity release compared to the natural variability of salinity in this system (Figure 7). The opposite trend was observed instead, where salinity decreased slightly throughout the day on February 7 at both the outfall and pier (Figure 7). The salinity at the pier does appear to be slightly higher than the outfall, which could be due to multiple factors. The first is that the use of different sensors led to an offset, however, it is unlikely that this would lead to an offset of 0.5. The second is that the pier is slightly closer to the inlet for the Bay from the Salish Sea, resulting in higher salinities. The third is that under normal WWTP operation, the salinity of the released water is slightly fresher than the surrounding water, considering it receives both fresh and saline water from PNNL facilities (Figures 4, 7). When analyzing the pH_NBS at the pier station over the week, there is a small increase during the discharge event, however, it is not significantly different from the average pH_NBS conditions occurring naturally within the system over this period (Figures 6, 7). Therefore, the alkalinity-enhanced seawater released at the outfall was likely diluted quickly near the outfall and did not affect the nearby pier station (Figure 7). This is further supported by the PLUMES2.0 model, which demonstrated that the pH signal dissipates within approximately 2.5 m from the outfall (Figure 8). In addition, strong currents near the outfall in Sequim Bay would promote mixing and dilution of the pH signal (Supplementary Figure S12). Before the alkalinity release, there were four regular wastewater releases from the WWTP that released warmer, less salty, and lower pH water to the Bay (Figure 7). Following the alkalinity release, there was a release from the WWTP on February 8 that released both wastewater from the labs as well as residual alkalinity treated water that was still in the GW Cells, which explains the peak in pH during this period (Figure 7). There were not any other releases recorded in this time series following the release on February 8 due to unrelated facility mechanical problems.

These field trials demonstrated the ability to release electrochemically generated alkalinity through an existing, pre-permitted coastal outfall with minimal to barely detectable influence on local water conditions. pH_NBS was successfully measured and controlled throughout the wastewater treatment system to ensure that pH of the effluent was limited to <9.0. There was minimal impact on water temperature and salinity while elevating the pH within the permitted thresholds (Figure 7). During normal WWTP operations, water has a longer residence time within the laboratory facilities, providing opportunities for water in the system to heat or cool relative to ambient seawater. During the February field trial, however, our continuous pumping of water substantially decreased this residence time, minimizing thermal differences between the discharge and receiving waters of Sequim Bay. It is important to note that the brine product stream was not added to the final released product in this study due to logistical reasons. Therefore, we did not expect to see shifts in salinity relative to natural seawater conditions during these releases. However, even if the brine product was reintroduced with the base product, it is unlikely that we could detect a signal due to its relatively small volume compared to the large volume of seawater it is diluted in (approximately only 0.2% of total volume).

The improved detection of alkalinity-enhanced seawater in February at the outfall was primarily due to a more optimal positioning of the sensors relative to the direct flow of the outfall (Figure 4) but may also be partly due to the larger volume of the alkalinity release in February compared to November (Figure 3). The difference between the two field trials in November and February demonstrates that even when the discharge volume increased by over 6-fold, there was still only limited detectability of increased pH to just outside the outfall pipe (Figures 5–7). In either case, the difference between ambient and OAE pH conditions is relatively small and may be difficult to capture beyond the outfall for MRV purposes (Figures 6, 7). Positioning sensors precisely within the direct flow of an outfall is logistically challenging, requiring divers to deploy a cage attached to the outfall, and optimal placement of sensors may vary with tidal conditions. Capturing the signal of a plume further from the outfall in a dynamic tidal system like Sequim Bay is even more challenging without a substantially larger treatment (i.e., higher pH and volume). These logistical and observational challenges highlight the need for modeling to support MRV, particularly for smaller-scale experiments and deployments.

During both field trials, there was no measured increase in pH_NBS by either the SAMI-pH or Hydrocat sensors at the nearby pier (Figures 5, 6). One possible reason for this is that the sensors installed at the pier station may not have been optimally positioned to capture the plume despite the fact that outgoing tides during the February experiment should have directed the plume towards the pier. However, based on data from the PLUMES2.0 model, the pH signal from the plume dissipates quickly within 2.5 m from the outfall due to fast dilution of the alkalinity-enhanced seawater by currents in this area (Figure 8; Supplementary Figure S12). These data, along with PLUMES2.0 simulations, indicate that the problem for detection was not necessarily sensor placements but the scale of the alkalinity release. Releasing seawater with higher alkalinity concentrations may increase the distance of detectability further from the outfall to allow for measurements further away (Figure 8). However, there is a potential tradeoff between the increased measurability of the plume and the increased localized impact of the alkalinity addition.

Evaluation of the above tradeoffs should include clear definition of research questions in the context of an OAE deployment, the range of natural variability in pH and carbonate chemistry inherent to a field site, and consideration of both potential positive and negative impacts of shifts in pH and carbonate chemistry on the marine communities that may be present within the perturbed volume of seawater around the outfall. We note that increasing the pH and alkalinity of the effluent also requires consideration of the potential for secondary precipitation of carbonate minerals, which is dependent on factors including temperature, salinity, carbonate chemistry, and suspended solids in the water column (Mucci, 1983; Zhong and Mucci, 1989; Pan et al., 2021; Wurgaft et al., 2021; Fuhr et al., 2022; Hartmann et al., 2023; Paul et al., 2024; Ringham et al., 2024; Suitner et al., 2024; Varliero et al., 2024). While many laboratory studies have investigated this precipitation, which is linked to both inefficiencies in alkalinity delivery and potential impacts on marine biology associated with increasing turbidity, there are few examples of methods for quantifying precipitation in OAE field studies (Moras et al., 2022). Therefore, where alkalinity is released under conditions approaching potential secondary precipitation thresholds, and until precipitation thresholds are better defined, we recommend collecting continuous measurements near the outfall for both carbonate chemistry and turbidity. It is important to note that most marketed turbidity sensors currently have an accuracy between 1% (Seabird) to 2% (YSI), which can still translate to relatively large error given the range in detectability (0–4000 NTU). Therefore, special care should be taken during calibration and data quality control checks. Still, these data will be useful for demonstrating compliance with water quality regulations, validating dilution models, and evaluating the impact of the alkalinity-enhanced seawater discharge on the local marine environment. Based on current literature, OAE has mixed impacts on marine species, even within individual groups of calcifiers (Ferderer et al., 2022; Gately et al., 2023; Jones et al., 2024; Bednaršek et al., 2025; Britton et al., 2025). For these field trials, we were unable to collect data to assess the impacts of the alkalinity-enhanced seawater delivered to the local marine species. However, we hypothesize that only organisms within direct proximity to the outfall would be impacted based on the limited extent and duration of the plume, along with the lack of evidence of secondary precipitation in situ.

The data from the PLUMES2.0 model exemplifies the value of leveraging near-field modeling capabilities to help inform field deployment strategies alongside regional ocean models used to quantify carbon removal resulting from OAE. Models can be used to build a better observation network by simulating a large number of model scenarios to explore when and where the best detectability occurs. As a result, we can develop a better MRV system for mCDR which is essential for the advancement of the technology. For example, the impact of tidal currents on the alkaline plume may limit detection by the deployed sensors. High-resolution forecasts of local current conditions could help inform stationary sensor placement for field trials. Likewise, model simulations could be used to program spatially-resolved monitoring routes by automated vehicles to track plumes in greater detail in more offshore waters. While PLUMES2.0 was used post-experiment to understand plume dynamics based on field data, it could very well be used in the future to determine the range of detectability from the outfall for future trials. In addition, broader regional models can help with tracking alkalinity plumes. The elevated pH and alkalinity and decreased pCO₂ resulting from alkalinity-enhanced seawater addition in these plumes may be measurable at the outfall, however CO₂ removal and storage occur over the timescales of air-sea equilibration in the broader oceanographic region (Jones et al., 2014; Bach et al., 2023; Ho et al., 2023; Wang et al., 2023). Measurement, reporting, and verification (MRV) for OAE must then rely on the combination of direct measurement of the rate and concentration of the alkalinity-enhanced seawater returned to the surface ocean, direct measurement of water quality parameters in the immediate vicinity of the outfall for permit compliance and impact monitoring, and ocean modeling to quantify carbon removal and storage (Fennel et al., 2023; Ho et al., 2023). Both near-field and regional model outputs can help inform field sensor deployment, which in turn would allow for better measurement of the alkalinity release for MRV and regulatory purposes. These models could also be used to help determine if a site is a viable option for OAE deployment based on local currents and tides. Laboratory experiments can supplement this work and provide insights into OAE by building a body of knowledge to assess the safety and viability of OAE in advance of field trials. Lab experiments can better capture processes that are not currently measurable due to signal-to-noise issues at the current scale of small field trials and help develop operational guidelines for MRV to improve our ability to evaluate the safety and efficacy of OAE, especially those focused around secondary precipitation kinetics and biological responses to OAE (Bach et al., 2023; Ho et al., 2023; Iglesias-Rodríguez et al., 2023; Riebesell et al., 2023; Bednaršek et al., 2025).

In summary, we leveraged existing outfall permits for the WWTP at PNNL-Sequim to release alkalinity-enhanced seawater within cautious pH thresholds, which is likely how operators with existing outfall permits (e.g., WWTPs, desalination plants, etc.) would approach early adoption of OAE deployments. However, other mCDR studies have shown that under similar seawater temperature and salinity conditions, we could release seawater at increased alkalinity and pH conditions (+1,400 μmol/kg and ∼9.5, respectively) with limited risk of secondary precipitation of carbonate minerals, particularly in regions where alkalinity-enhanced seawater discharges would be rapidly diluted into the surrounding environment (Suitner et al., 2024; Suitner et al., 2025). It is important to note that this is true for the alkaline feedstock used in this study and may not be applicable to other forms of alkaline feedstock. For example, lower precipitation thresholds have been observed when using solid mineral dispersal versus aqueous solutions, likely due to the increased number of precipitation nuclei (Moras et al., 2022; Hartmann et al., 2023; Moras et al., 2024; Suitner et al., 2024). The facilities at PNNL-Sequim could be leveraged for further field testing and significant advancements in mCDR research with appropriate discharge permits that allow for the release of increased alkalinity-enhanced seawater. The WWTP at PNNL-Sequim is relatively small compared to typical municipal water treatment facilities in Washington state that may operate at >1,000x the capacity of this facility. A quick calculation based on the volume of alkalinity output by the PNNL facility during the February field trial release indicated that the amount of alkalinity released (∼251 mol NaOH) could account for about 8.9 kg of equivalent CO₂ for this one experiment, based on the conversion of ∼0.8 mol CO₂ captured per mol alkalinity released in the form of NaOH (Wang et al., 2023; Khangaonkar et al., 2024; Ringham et al., 2024). Daily repetition of this experiment over the course of a year could result in sequestration of ∼3.2 metric tons of CO₂ per year, which is about 70% of the yearly emissions from one typical US car (US EPA, 2025). The Ebb Carbon BPMED system at PNNL-Sequim could produce enough NaOH to sequester ∼100 tons of CO2 per year, enough to offset around ∼22 cars per year (US EPA, 2025). However, discharge of this volume of alkalized seawater is constrained by our capacity for onsite dilution to allow for release of an effluent pH_NBS < 9.0. While PNNL-Sequim’s WWTP would need permits that allow for release at higher pH to have a broader impact in carbon removal, it has high value as a research and development test bed in OAE.

Paired with larger WWTP or industrial facilities that discharge large volumes of seawater, electrochemically derived aqueous alkalinity provides a valid approach among many in the CDR toolbox. Even if constrained to release seawater of pH_NBS < 9.0, the release of alkalinity-enhanced seawater from larger facilities would begin to increase ocean carbon uptake and regionally offset ocean acidification (Khangaonkar et al., 2024). However, we note that there is a disconnect between the intention of NPDES permits, namely, to regulate, control, and minimize the impact of pollutants on the natural environment, and the intention of OAE, which is to perturb the natural environment through the addition of alkalinity (Steenkamp and Webb, 2023). While some NPDES permits may technically allow for OAE, we cannot extrapolate this work to imply that all NPDES-permitted outfalls will be allowed to intentionally discharge alkalinity (Murthy et al., 2025). It is also important to note the disconnect between pH reporting between these two communities. From a regulatory standpoint, the NBS scale for pH is commonly output by the lower cost sensors and reported for compliance and monitoring purposes. However, the researchers studying mCDR OAE or ocean acidification typically report on total scale, as it is commonly measured by oceanographic instruments. This is necessary to keep in mind for future research experiments in OAE so that results are communicated accurately to the regulatory community.

We recommend that future research should continue to refine constraints, monitoring, and modeling approaches for ocean alkalinity enhancement that enhance the growing body of research. Field demonstrations of alkaline releases can be leveraged to more easily acquire appropriate NPDES permits for OAE deployment at coastal facilities in the future, such as that of Ebb Carbon’s upcoming research pilot with an increased effluent pH limit in Port Angeles, WA. This work will allow for greater scaling in OAE field trials, which could be useful for integrating these systems with coastal facilities going forward. When possible, high-accuracy continuous measurements should be utilized near the outfall for field trials, however, we recognize that this may not be sustainable for regular compliance at many facilities. Therefore, calibration and comparison with discrete samples should be done at regular intervals when using less accurate but more cost-efficient sensors to account for any offset between the two. In addition, we think this study would have benefitted from additional post-deployment calibration checks and recommend them for future field work campaigns. Finally, we emphasize that the limits of detection of alkaline signals in dynamic coastal environments require improvement in both dilution modeling, to assess the movement and potential near-field impacts of alkaline plumes, and regional modeling, to quantify resulting carbon dioxide removal. These tools are already valuable for OAE MRV, however, improving parameterizations of variables like secondary precipitation and gas transfer velocity for air-sea gas exchange would improve their accuracy. The field of mCDR and OAE is quickly expanding, and we hope that the rapid reporting of this work can help inform the community as new research evolves.