High-resolution observations of total alkalinity (TA) and dissolved inorganic carbon (DIC) across the land-ocean interface are essential for riverine-based alkalinity enhancement for marine carbon dioxide removal. The excess alkalinity [TA-DIC] and the TA:DIC ratio (R) serve as useful proxies for "buffering capacity" of the water. Here, we present results from 16 sampling campaigns between Sept 2024 to Sept 2025 for >40 stations from Xichong River to nearby Dapeng Bay of Shenzhen, encompassing river (including wastewater effluent), estuary, groundwater (beach monitoring wells and piezometers), and bay water. We conclude that sea-bound riverine excess alkalinity and R ratios are highly anthropogenically perturbed, and in turn, influences very near shore subtropical coastal embayment DIC system.
In 6 stations from upstream to downstream river, with discharge ranging from 4,047 to 246,603 m³/d and fresh to brackish waters (salinity 0.1-10.5), excess alkalinity and R were slightly higher in the dry season (97±252 μmol/L, 1.11±0.24, Sal=1.1±1.5, n=21) than in the wet season (56±682 μmol/L, 1.10±0.61, Sal=0.7±1.7, n=36), influenced by varied waste water inputs-notably a subway construction site effluent with excess alkalinity (231±816 μmol/L, R=1.23±0.51, n=8) and a waste water treatment plant effluent (1,409-4,231 m³/d) with an alkalinity deficit (-327±102 μmol/L, R=0.80±0.07, n=8). In 7 estuarian stations, TA and DIC values fall largely on a linear mixing line with salinity. Estuary excess alkalinity and R were also higher in the dry season (147±143 μmol/L, 1.09±0.09, Sal=11±10, n=33) and lower during the wet (92±147 μmol/L, 1.04±0.08, Sal=15±12, n=38).
In 5 beach monitoring wells (2-2.6m depth, 50-100m from shoreline), excess alkalinity and R exhibited strong variabilities and were not significantly different during dry (76±114 μmol/L, 1.03±0.03, Sal=7.2±6.7, n=33) and wet (60±282 μmol/L, 1.03±0.08, Sal=2.7±6.5, n=21) seasons. Additionally, shallow groundwater (8 stations, <1 m depth) sampled by mini-piezometers along the beach shoreline showed higher excess alkalinity (149±88 μmol/L) and R (1.07±0.03) during the dry season (Sal=27±7.3, n=31) than those at 82±109 μmol/L and 1.04±0.05 in the wet season (Sal=18.5±10.5, n=40).
Seawater in Xichong embayment, inner Dapeng Bay (5 stations, each sampled at surface and 4-12 m depth within 2 km of shoreline) exhibited the most stable excess alkalinity (~248 μmol/L) and R (~1.13), slightly higher and more variable in the wet season (264±63 μmol/L, 1.14±0.04, Sal=30±2.7, n=42) than in the dry (233±36 μmol/L, 1.12±0.02, Sal=31.6±0.3, n=44). Seawater from outer Dapeng Bay displays excess alkalinity of 189±124 μmol/L in wet season while 69±65 μmol/L in the dry, both significantly lower than the global mean (239 μmol/L, R=1.12).

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