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winter-dry with precipitation delivered sizes, while avoiding rivers that had major ing in two samples with MPN/100ml >1000
both by trade-wind showers and by larger dams (Figs. 2 and 3N). See the GSA Data (CU-107 and 110) did not identify any human-
tropical storms. Repository for detailed methods. Our anal- sourced bacteria; rather, the bacteria in sam-
1
The diverse geology of Cuba reflects its ysis assumes that the concentration of cat- ple CU-110 were identified as being of ungu-
tectonic setting at the boundary of the ions and anions we measured are represen- late origin, and no specific source could be
North America and Caribbean plates. tative of annual average values (Godsey et determined for bacteria in CU-107.
Central Cuban basement lithologies include al., 2009). There are numerous correlations between
accreted igneous rocks, sediments (clastic, anions and cations in our river water sam-
carbonate, and evaporite) formed along pas- RESULTS ples (Table S5 [see footnote 1]). Na and Cl
sive margins, obducted ophiolite, and island River water samples from central Cuba are positively correlated (p < 0.01) as well
arc rocks (Iturralde-Vinent et al., 2016). contain high concentrations of dissolved as Na and HCO , F, SO , NO , K, Ca, Br, Ti,
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4
3
This basement is unconformably overlain material (Figs. 3 and 4). Conductivity and As, Rb, Sr, Ba, and U (p < 0.05, all positive,
by slightly deformed, younger marine and total dissolved load were high (130–1380 Fig. 4). These elements are also correlated
terrestrial sedimentary rocks (Iturralde- μS/cm and 117 to over 780 mg/L, respec- to one another positively and significantly.
Vinent, 1994). Where river water has inter- tively, Tables S1 and S2 [see footnote 1]); In addition, Mg is positively correlated to
acted with these diverse rocks, surface stream water, except that sampled from for- SiO , V, Cr, and Ni (p < 0.05). NO is posi-
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water chemistries should reflect the compo- ested catchments, was turbid. Sample pH tively correlated with conductivity.
sition of underlying rock units. was near neutral to slightly alkaline with Four of the 25 samples (CU-120, -121,
Agriculture has been practiced in Cuba for high values of bicarbonate alkalinity (65– -122, and -132), all collected in the north-
centuries. Indigenous people cultivated cas- 400 mg/L). As, Ba, Cr, Mn, Ni, Sr, and U western part of the field area, are geochemi-
sava, yucca, and maize (Cosculluela, 1946). were present in some or all of the Cuban cally distinct (Figs. 3, 4, and 5). These sam-
Spanish colonization from 1492 brought river waters we analyzed, in all cases at lev- ples have the highest or nearly highest Cl,
slaves, large-scale sugar agriculture, and els below drinking water standards (Table SO , Br, NO , and Na concentrations, field
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cattle farming (Zepeda, 2003). Following S3 [see footnote 1]). Dissolved oxygen mea- conductivity, and TDS (Fig. 4, red symbols)
Cuba’s independence from Spain in 1898, sured in the field ranged from 59% to 145% in the sample set. These are four of only
sugar production in Cuba quadrupled under (average 97%). Using basin-specific pre- five samples to contain low but measurable
U.S. influence (Whitbeck, 1922). When cipitation (Fig. 3), along with run-off esti- As (1.0–1.4 ppb). They plot in a distinct
Cuba allied with the Soviet Union in 1959, mates (Beck et al., 2015, 2017) and total dis- zone of the Piper diagram (Fig. 5) and also
industrialization of the sugar industry to solved solids (TDS) from each Cuban water have higher Rb, Sr, Ba, and U concentra-
increase yields and exports became a central sample, we estimate chemical weathering tions (1.8–4.3 ppb) than other Cuban river
goal (Pérez-López, 1989). By the 1980s, rates between 42 and 302 t km y with a water samples. Three of the four samples
–1
–2
Cuba boasted the most mechanized agricul- mean of 161 ± 66 t km y . contain >115 mg/L Ca and high concentra-
–1
–2
tural sector in Latin America (Febles- Dissolved organic carbon (DOC) was tions of Na, Cl, and SO . These four sam-
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González et al., 2011); however, the collapse highly variable, ranging from <1 mg/L to ples were collected near one another and
of the Soviet Union in 1991 catalyzed Cuban 9 mg/L (Table S4 [see footnote 1]). Total drain the same bedrock map unit (post-
adoption of reduced tillage, organic soil dissolved nitrogen (TDN) ranged from Eocene marine sediment). One (CU-122)
amendments, the use of cover crops, and the <0.1–1.5 mg/L (mean = 0.76 mg/L); on aver- drains mostly wetland while the others
replacement of fuel-hungry tractors with age 60% was present as nitrate (range 24%– drain dominantly agricultural catchments.
domesticated draft animals, including horses 93%). Nitrate values measured in the field
and oxen (Gersper et al., 1993). and then in the lab several weeks later are DISCUSSION/INTERPRETATION
Surface water biogeochemical monitor- well correlated. Nitrite was present in all
ing in central Cuba has focused mainly on samples, averaging 1.2 mg/L (0.37 mg/L of Bedrock Controls Central Cuban
reservoirs. In central Cuba, water chemis- N). DOC/TDN ratios also vary widely, from River Water Chemistry
try data (1986–2005) from four reservoirs, 1.3 to 14.8. Anion concentrations decreased In central Cuba, river water composition
representing two river systems and four in the order HCO > Cl > SO > NO > HPO 4 and TDS covary with rock types (Figs. 3 and
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3
3
basins with varied geology (Betancourt et > NO > Br > F. 4D) suggesting a close connection between
2
al., 2012) showed that the primary control The anion orthophosphate (as P) was mea- river water chemistry and underlying rock
on major ion concentration is rock weather- sured both in the field (0.1–0.8 mg/L) and lab units. For example, high concentrations of
ing upstream; there was no statistically sig- (0.4–0.5 mg/L); field and lab analyses were Ca, Mg, and alkalinity in most samples are
nificant difference in water chemistry positively correlated. Cations decreased on consistent with the mapped presence of car-
between dry and rainy seasons in three of average in the order Ca > Na > Mg > Si > K. bonate rocks in most sampled drainage
the four basins. E. coli bacteria were found in all samples, basins (Fig. 3). Distinct anion, cation, and
In August 2018 (the wet season), we col- and most samples (20/24) contained enough trace metal compositions of rivers draining
lected water samples from 25 river basins in bacteria to be deemed unsafe for recreational four (CU-120, -121, -122, -132) watersheds in
central Cuba. We selected these sites to use according to World Health Organization the NW quadrant of our field area and under-
encompass a range of land uses, underlying criteria (Most Probable Number (MPN) > lain by marine sediments (French and
upstream rock types, discharges, and basin 127/100 ml). Genetic microbial source trac- Schenk, 2004) suggest dissolution of evap-

1 GSA Data Repository item 2020097, data tables and extended methods, is available online at https://www.geosociety.org/datarepository/2020.
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