Global land-ocean linkage: direct inputs of water and associated nutrients to coastal zones via submarine groundwater

1. 20th SWIM 24. June 2008 Global land-ocean linkage: direct inputs of water and associated nutrients to coastal zones via submarine groundwater discharge (SGD) Hans Dürr(1), R. van Beek(1), C.P. Slomp(2), H. Middelkoop(1), M. Bierkens(1) h.durr@geo.uu.nl (1) Department of Physical Geography, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands (2) Department of Earth Sciences – Geochemistry, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands

2. Effects of nutrient enrichment (eutrophication) of coastal waters more algal blooms red tide (harmful algal bloom) foam

3. Rationale • Higher nutrient inputs (N, P) from rivers to coastal waters + estuaries, inverse trend (decrease) due to damming for Si • Global nutrient fluxes at the land-ocean interface: Terrestrial controls, impact on coastal waters and response to global change. Spatially explicit, multiple nutrient forms (dissolved / particulate, inorganic / organic) • Box modelling (Slomp & van Cappellen 2004) has shown potential impact on coastal zone of nutrient SGD at global scale • Typically: Groundwater N and P flux = total SGD flux x nutrient concentration, but decisive for net balance: fresh GW flux x nutrient concentration ( local scale effects of subterranean estuary) • Submarine Groundwater Discharge (SGD) of nutrients: spatially explicit not yet estimated at global scale, comparison to river inputs (water discharge globally ~5-10% of river discharge)?

4. groundwater input Proximal coastal ocean N:P ~ 14:1 river input eolian input N denitrification Phytoplankton (mostly N-limited) Redfield N:P = 16:1 export to continental shelf N:P mostly >>> 16 recycling burial • Fresh SGD: mostly N, less P (elevated velocity seeps in shallow, oxic systems: P removal, conservative behaviour of NO3), could drive systems to P-lim. • Only in cases when both GW and sediment are anoxic and upon saltwater intrusion: N/P ratio of SGD < Redfield • Deep GW supply mostly minor & not contaminated

5. QChannel PREC Epot River ice RS Tw TA Snow/Rain RS LH(Epot) TA Snow/Rain QChannel Model structure Epot PREC Each cell: • Conceptual model partly based on the HBV model • Climate variables are from the ERA40 data set • Transfer / Routing of runoff to the drainage network Canopy Eact QDR T Snow cover Store 1 P QSf Store 2 For baseflow here: • Not fully calibrated yet • Energy balance + lakes will be considered in later stages • Monthly values  annual means Q QBf P Store 3 0,5° grid Courtesy R. van Beek

6. Concept of groundwater flow • Kraaijenhof-Van de Leur (1958) • Boussinesq – Dupuit assumptions to calculate the reservoir coefficient kr where: k is the hydraulic conductivity (L·t-1 e.g., m/day), D is the aquifer thickness (L, 50m assumed), f is the drainable porosity (obtained from lithology + some Holdridge climate data) (dimensionless, 0-1) and B is the aquifer width (L).Using various global data sets Courtesy R. van Beek

7. Result: Residence time of Groundwater • Long residence times: • Highly weathered soils • Tropics • Model performance (no full calibration yet): • Good in most temperate and tropical regions • Weaker in Arctic + semi-arid regions: no evaporation from surface waters yet, snowmelt driven by mean monthly temp. • Next step – full calibration on low flow conditions Courtesy R. van Beek

8. Local groundwater discharge High in: Some tropical regions, esp. SE-Asia / Indonesia; W Canada, Florida; Ex.: Patos Lagoon Low: Arctic, but % contribution to total flow is high Courtesy R. van Beek

9. Next steps, including nutrients • Coastal water divide where baseflow = SGD?? • Human GW extraction? • Salt-water intrusion sites (low-lying areas)?! • Range of GW nutrient concentrations for DIN ~NO3 • Test nutrient attenuation scenarios, e.g., based on half-life approaches • Coupling to information from landuse and population

10. Coastal ribbon definition for SGD: distance to water divide at coast • Problem : • what is a stream ??? • every catchment basin that has a cumulated upstream area > xx km2  testing possible last gauging station subgrid variability 0,5° grid cell

11. Groundwaterabstraction • Water extraction: • country based GW abstraction from IGRAC / TNO • linked to total water use and population numbers from Vörösmarty et al. Courtesy Rens van Beek

12. Range ofGW nutrientconcentrationsin different landusesettings (NO3) Agricultural Urban Misc. Undevel. land USGS Summary data

13. High Groundwater Nitrate sites Data from IGRAC / TNO

14. Potential hotspots with potentially elevated N input • Caution: • Very local phenomena not detected • Total Groundwater flow in coastal cells, not SGD • GW abstraction not considered • Saltwater intrusion not considered • Effect of residence time in GW on nutrient concentrations Some study sites for SGD including nutrients

15. Rivers DIN-NEWS empirical model: Dumont et al. 2005 Similarities in regions, local differences Groundwater

16. Conclusions: first steps towards • Spatially explicit estimates of SGD at global scale, using baseflow estimates from new global hydrological model • Potential hot spots of freshwater nutrient SGD • SE Asia (esp. Indonesia) + Central America: hot spots for both river and GW fluxes (high baseflow, high runoff and high levels of human activity) – exact locations and time scales may be different • Field studies needed in high ‘risk’ areas, e.g. (sub-)tropical areas of Africa, S America and SE Asia – some studies now underway • Examples have been shown to include amendments such as GW abstraction, GW quality data, landuse + population data

17. As source of ‘new’ nutrients, especially N (less P), freshwater SGD is potentially important for coastal nutrient cycling (N-limitation of most coastal waters) at global scale (strong response to human impact)

18. Thank you ! Questions ? Hand-outs available, ask anytime or email: h.durr@geo.uu.nl

20. Salt-water intrusion: CaHCO3 freshwater is replaced by NaCl and SO42- rich sea water Na+ Ca2+ Ca2+ Na+ Na+ Ca2+ Ca2+ Na+ • Na2+ replaces Ca2+, NH4+ • Cl- or SO42- replaces PO4 • - Increased degradation of organic matter (Nyvang, 2003) • Increased sulfate reduction & reduction of Fe-oxides by S2- • NH4 and PO4 release to the groundwater