Human-induced changes in carbon fluxes across the land-ocean interface can influence the global carbon cycle, yet the impacts of rapid urbanization and establishment of wastewater treatment plants (WWTPs) on coastal ocean carbon cycles are poorly known. This is unacceptable as at present ∼64% of global municipal wastewater is treated before discharge. Here, we report surface water dissolved inorganic carbon (DIC) and sedimentary organic carbon concentrations and their isotopic compositions in the rapidly urbanized Jiaozhou Bay in northeast China as well as carbonate parameters in effluents of three large WWTPs around the bay. Using DIC, δC and total alkalinity (TA) data and a tracer model, we determine the contributions to DIC from wastewater DIC input, net ecosystem production, calcium carbonate precipitation, and CO outgassing. Our study shows that high-DIC and low-pH wastewater effluent represents an important source of DIC and acidification in coastal waters. In contrast to the traditional view of anthropogenic organic carbon export and degradation, we suggest that with the increase of wastewater discharge and treatment rates, wastewater DIC input may play an increasingly more important role in the coastal ocean carbon cycle.

China has achieved significant progress on wastewater treatment and aquatic environmental protection. However, leakage (in- and exfiltration) of sewer systems is still an issue. By using the statistical data of water and wastewater in 2016 in China, and the person loads (PLs) of water and wastewater in Singapore, the leakage fractions of hydraulic flow, organic carbon (COD), nitrogen (N) and phosphorus (P) mass loading, and in-sewer COD biological removal in the sewer systems of China (except Hong Kong, Macau and Taiwan), Shanghai, Guangzhou and Beijing were reported for the first time. The fractions of hydraulic flow infiltration (13%, Shanghai and Guangzhou) and exfiltration (39%, China) were calculated. Except Beijing, whose sewer networks are under appropriate management with small leakage fractions, the exfiltration fractions of COD (including in-sewer biological COD removal) ranged from 41% (Shanghai) to 66% (China) and averaged 55%; N ranged from 18% (Shanghai) to 48% (China) and averaged 33%; and P ranged from 23% (Shanghai and Guangzhou) to 44% (China) and averaged 30%. The exfiltrated sewage, COD, N and P not only wastes resources, but also contaminates the aquatic environment (especially groundwater) and contributes to ‘black and odorous water bodies’. In- and exfiltration in the sewer network leads to low influent COD concentration, C/N ratio and high inorganic solids and inert particulate COD concentrations of many municipal wastewater treatment plants (WWTPs) causing high cost for nutrient removal, poor resource recovery, additional reactor/settler volume requirement and other operational problems. Therefore, tackling sewer leakage is of primary importance to today's environment in China. Recommendations for the inspection of sewer systems and the rehabilitation of damaged sewers as well as the development of design and operation guidelines of municipal WWTPs tailored to the specific local sewage characteristics and other conditions are proposed.

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Understanding the natural variability of pH and aragonite saturation state (Ωarag) is important for assessing ocean acidification (OA) impacts especially in the coastal ocean since anthropogenic CO2 increase‐induced OA is often superimposed by their natural variability. Here, we report the seasonal variability of sea surface pH and Ωarag from spring to summer in the Jiaozhou Bay (JZB) and compare their controls based on two cruises conducted in April and August 2018. Results show that sea surface pH on the NBS scale slightly increases from 8.10 ± 0.05 in spring to 8.13 ± 0.04 in summer, whereas surface Ωarag substantially increases from 2.05 ± 0.18 in spring to 3.34 ± 0.25 in summer. The difference in pH and Ωarag seasonal increase is related to the contrasting temperature effects on them, which can be divided into the first temperature effect associated with acid‐base equilibrium of the CO2 system and the second temperature effect associated with CO2 solubility‐driven air‐sea exchange. The two temperature effects have opposite influences on pH, canceling each other and causing a relatively small seasonal variability of pH, while they have consistent influences on Ωarag, reinforcing each other and causing a relatively large variability of Ωarag. Also, through both qualitative analyses and a 1‐D model, we identify the processes controlling the seasonal variability of pH and Ωarag. We find air‐sea exchange dominates the seasonal variability of pH and Ωarag in nearshore areas, while biological production is the most important in the central part of the JZB.