Origin and chemical and isotopic evolution of dissolved inorganic carbon (DIC) in groundwater of the Okavango Delta, Botswana

Figures & data

Figure 1. Map of the Okavango Delta, Botswana, showing selected sample locations (●: shallow groundwater, ■: deep groundwater, ♦: island groundwater and ▲: surface water). Map modified from McCarthy et al. (2012).

Table 1. Statistical summary of data for selected parameters for shallow groundwater, deep groundwater, island groundwater and surface water from the Okavango Delta, Botswana. Min.: minimum; Max.: maximum; SD: standard deviation. DIC: dissolved inorganic carbon; TDS: total dissolved solids.

Parameter	Water type	Mean	Min.	Max.	SD	No. of samples
DIC (mg C/L)	Surface water	9.1	0.6	16.3	38	38
	Shallow groundwater	23.2	10.1	153.6	24	24
	Island groundwater	192.1	22.2	847.5	21	21
	Deep groundwater	67.6	32	93.2	14	14
δ^13CDIC (‰)	Surface water	–7.5	–12.3	–1.9	38	38
	Shallow groundwater	–10.1	–12.9	–7.8	23	23
	Island groundwater	–7.2	–14.2	–1.1	21	21
	Deep groundwater	–9.3	–11.8	–5.3	16	16
δ^18O (‰)	Surface water	0.7	–7.2	17.4	92	92
	Shallow groundwater	0.8	–4.9	4.5	7	7
	Island groundwater	1.7	–0.8	4.5	28	28
	Deep groundwater	–3.5	–7.8	2.2	35	35
δD (‰)	Surface water	–7	–47	73	38	38
	Shallow groundwater	–4	–27	7	41	41
	Island groundwater	–1	–22	14	26	26
	Deep groundwater	–28	–54	3	34	34
TDS (mg/L)	Surface water	157	0	4900	72	72
	Shallow groundwater	188	64	1676	29	29
	Island groundwater	3020	29	21 038	116	116
	Deep groundwater	1154	228	9266	41	41
pH	Surface water	6.7	4.9	10	60	60
	Shallow groundwater	6.3	5.8	7.1	24	24
	Island groundwater	7.6	5.6	9.6	115	115
	Deep groundwater	7.6	6.2	8.6	39	39
K (mg/L)	Surface water	174.1	1	8700	61	61
	Shallow groundwater	5.1	1	9.9	25	25
	Island groundwater	167	2.8	1365	114	114
	Deep groundwater	12.6	2	48	37	37
Na (mg/L)	Surface water	2162.8	1.7	115 000	61	61
	Shallow groundwater	11	3.9	25.9	25	25
	Island groundwater	1287.3	4.8	10 209.1	115	115
	Deep groundwater	263	28	851.7	37	37
Ca (mg/L)	Surface water	7.3	1	24	61	61
	Shallow groundwater	30.3	2	145.1	113	113
	Island groundwater	30.3	2	145.1	113	113
	Deep groundwater	22.3	0.6	84.5	37	37
Mg (mg/L)	Surface water	2.6	0.7	19	61	61
	Shallow groundwater	5	1.9	44.2	25	25
	Island groundwater	15	0.6	128	114	114
	Deep groundwater	10.1	0.1	75.8	37	37
Alkalinity (mg/L)	Surface water	245	9	6319	61	61
	Shallow groundwater	73	40	311	24	24
	Island groundwater	4027	27	38 586	109	109
	Deep groundwater	405	137	1026	38	38
Cl (mg/L)	Surface water	164.3	0.1	6400	57	57
	Shallow groundwater	1.8	1.2	2.5	25	25
	Island groundwater	158.9	0.2	1968.2	90	90
	Deep groundwater	135.5	1.2	1596.7	38	38
Sulphate (mg/L)	Surface water	38.3	0	1700	55	55
	Shallow groundwater	1.1	0.1	10.3	25	25
	Island groundwater	92.8	0.1	1528.9	89	89
	Deep groundwater	150.8	4.6	1162	38	38

Figure 2. Plot of dissolved inorganic carbon (DIC) vs the stable carbon isotope composition of DIC (δ¹³C_DIC) for shallow groundwater, deep groundwater, island groundwater and surface water.

Figure 3. Conceptual model showing the pathways and processes that affect dissolved inorganic carbon (DIC) in the river water, island groundwater and the unsaturated zone prior to recharge of groundwater in the Okavango Delta, Botswana.

Figure 4. Plot of the stable oxygen (δ¹⁸O) vs stable hydrogen (δD) isotopic composition for shallow groundwater, deep groundwater, island groundwater and surface water. Also shown is the Local Meteoric Water Line (LMWL) constructed from Maun rain, the Global Meteoric Water Line (GMWL; Craig and Gordon, 1965) and the Okavango Delta evaporation line (ODEL; Atekwana et al. 2016).

Figure 5. Piper plot showing the proportions of major cations and anions in shallow groundwater, deep groundwater, island groundwater and surface water.

Figure 6. Cross plots of (a) TDS vs stable isotopic composition of oxygen (δ¹⁸O), (b) TDS vs total cations (Ca²⁺ + Mg²⁺ + Na⁺ + K⁺), (c) TDS vs alkalinity and (d) TDS vs DIC for shallow groundwater, deep groundwater, island groundwater and surface water. TDS: total dissolved solids; DIC: dissolved inorganic carbon.

Figure 7. Cross plots of (a) alkalinity vs pH, (b) DIC vs pH, (c) alkalinity vs Ca²⁺, (d) DIC vs Ca²⁺, (e) alkalinity vs Mg²⁺, (f) DIC vs Mg²⁺, (g) alkalinity vs Na⁺ and (h) DIC vs Na⁺, for shallow groundwater, deep groundwater, island groundwater and surface water.

McCarthy, T.S., et al., 2012. Island forming processes in the Okavango Delta, Botswana. Geomorphology, 179, 249–257. doi:10.1016/j.geomorph.2012.08.016

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Atekwana, E.A., et al., 2016. Effect of evapotranspiration on dissolved inorganic carbon and stable carbon isotopic evolution in rivers in semi-arid climates: the Okavango Delta in North West Botswana. Journal of Hydrology: Regional Studies, 7, 1–13.

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