Surface controls on the characteristics of natural CO2 seeps: implications for engineered CO2 stores

CO₂ injected into rock formations for deep geological storage must not leak to surface, since this would be economically and environmentally unfavourable, and could present a human health hazard. In Italy natural CO₂ degassing to the surface via seeps is widespread, providing an insight into the various styles of subsurface ‘plumbing’ as well as surface expression of CO₂ fluids. Here we investigate surface controls on the distribution of CO₂ seep characteristics (type, flux and temperature) using a large geographical and historical data set. When the locations of documented seeps are compared to a synthetic statistically random data set, we find that the nature of the CO₂ seeps is most strongly governed by the flow properties of the outcropping rocks, and local topography. Where low‐permeability rocks outcrop, numerous dry seeps occur and have a range of fluxes. Aqueous fluid flow will be limited in these low‐permeability rocks, and so relative permeability effects may enable preferential CO₂ flow. CO₂ vents typically occur along faults in rocks that are located above the water table or are low permeability. Diffuse seeps develop where CO₂ (laterally supplied by these faults) emerges from the vadose zone and where CO₂ degassing from groundwater follows a different flow path due to flow differences for water and CO₂ gas. Bubbling water seeps (characterized by water bubbling with CO₂) arise where CO₂ supply enters the phreatic zone or an aquifer. CO₂‐rich springs often emerge where valleys erode into CO₂ aquifers, and these are typically high flux seeps. Seep type is known to influence human health risk at CO₂ seeps in Italy, as well as the topography surrounding the seep which affects the rate of gas dispersion by wind. Identifying the physical controls on potential seep locations and seep type above engineered CO₂ storage operations is therefore crucial to targeted site monitoring strategy and risk assessment. The surface geology and topography above a CO₂ store must therefore be characterized in order to design the most effective monitoring strategy.     

Hydrogeological Knowledge from Below: Water Expertise as a Republican Common in Early-Modern Venice**

This essay looks at early-modern Venice hydroculture as a case of episteme from below. The forms of water knowledge it developed were multilayered and collective in their essence and solidly rested on a social experiential basis that was rooted in labour (especially fishing) and practices (especially water surveying and engineering). In accordance with the city's republican esprit (and correspondent political values), its episteme emerged as the encounter and negotiation between various institutions and groups: the fishermen of San Niccolò in Venice, the practitioners of the water magistrature and political authorities. This essay explores the institutional settings of this water culture, seen as an instance of bottom-up epistemic construction. It especially addresses three historical instances: firstly, a seventeenth century program to map public waters in order to block their alienation for private fish farming; secondly, water officers’ interviews with fishermen aimed to assess the state of the lagoon hydromorphology and, thirdly, fishing regulations. Venice communitarian and circular forms of knowledge production are here contrasted to an opposite paradigm, which was embodied by the Galileian mathematician and Rome courtier, Benedetto Castelli. His interactions with the Republic of Venice on water management and his approach to hydraulic problems are revealing of an elitist and abstract understanding of scientific knowledge that guided political decisions from above without taking in any consideration the opinions of the ‘vulgar’. While his science was the expression of a top-down political epistemology, Venetian water knowledge was more egalitarian. It left room for exchange, inclusiveness and bottom-up codification; it valued the gathering of different experiences (including the fishermen's practical knowledge of their waters) and rested on a concrete and systemic (organicist) understanding of natural-anthropic processes.     

Obscure oases: natural,cultural and historical geography of western Queensland's Tertiary sandstone springs

The distribution of surface water dictates human and animal activity in arid zones. Although typically small, hidden, and inaccessible, springs and wells fed by local aquifers were the only sources of reliable water across vast areas of inland Australia until the last century. Compared to larger, more accessible water sources such as riverine waterholes and Great Artesian Basin discharge springs, their history is sketchy and poorly documented, although rich in intrigue and mythology. Since the expansion of artificial waters and motorised transport, many of these small oases have been forgotten, and their location and permanence are now less well‐known than for thousands of years. We examine the distribution, hydrogeology, cultural history, and biological values of Tertiary sandstone springs in western Queensland based on a review of historical literature, interviews with long‐term residents, and extensive field surveys. One hundred and sixty springs were documented, and nearly 40% of these have declined in flow or become inactive since pastoral settlement for reasons that are not well understood. While their decline in some areas seems related to shallow bores sunk into their local aquifers, it is possible that some smaller springs owed their existence to regular human maintenance. Others are probably naturally dynamic over decadal time scales. This study documents an almost‐forgotten aspect of Queensland's natural and cultural history.     

Tectonic evolution of a Paleozoic thrust fault influences the hydrogeology of a fractured rock aquifer,northeastern Appalachian foreland

In polyorogenic regions, the superposition of structures during a protracted tectonic history produces complex fractured bedrock aquifers. Thrust‐faulted regions, in particular, have complicated permeability patterns that affect groundwater flow paths, quantity, and quality. In the Appalachian foreland of northwestern Vermont, numerous bedrock wells that are spatially related to the Paleozoic Hinesburg thrust have elevated naturally occurring radioactivity and/or low yields. The association of groundwater quality and quantity issues with this thrust was a unique opportunity to investigate its structural and hydrogeologic framework. The Hinesburg thrust juxtaposed metamorphic rocks of the hanging wall with sedimentary rocks of the footwall during the Ordovician. It was then deformed by two orthogonal Devonian fold sets and was fractured during the Cretaceous. Median well yields in the hanging wall aquifer are significantly lower than those of the footwall aquifer, consistent with the respective permeability contrast between metamorphic and carbonate rocks. For wells drilled through the Hinesburg thrust, those completed closest (vertically) to the thrust have the highest median yields, whereas others completed farther below have yields in the footwall range. The geochemical signature of the hanging wall and footwall aquifers correlates with their whole‐rock geochemistry. The hanging wall aquifer is enriched in alpha radiation, Na+K‐Cl, Ba, and Sr, whereas the footwall aquifer is enriched in Ca‐Mg‐HCO₃ and alkalinity. Wells that penetrated the Hinesburg thrust generally have hanging wall geochemical signatures. A simple hydrogeologic model for the permeability evolution of the Hinesburg thrust involves the ductile emplacement of a low‐K hanging wall onto a high‐K footwall, with subsequent modification by fractures.     

Hydrogeological Modelling of Water-Level Changes in an Area of Archaeological Significance: A Case Study from Flag Fen,Cambridgeshire, UK

Concern exists regarding the long-term viability of the internationally important archaeological remains of the Flag Fen site near Peterborough. This paper describes a hydrogeological numerical groundwater model which has been used to understand the groundwater situation with regard to the archaeology, including drainage impacts, and also to explore future scenarios. Observed and modelled groundwater levels have been interpreted using three zones: the ‘dry’ zone 1 above the seasonal maximum water table; zone 2 of seasonal water table fluctuation; and a deeper zone 3 of permanent saturation. Archaeological wood is best preserved in Zone 3. Much of the Bronze Age wooden structure is located within Zone 2 or above. The hydrological conditions are therefore not ideal for the long-term in situ preservation of the material. Groundwater modelling indicates that the main factor controlling groundwater levels in the area is artificial drainage. The approach would be applicable to other wet-preserved archaeological sites.     

遗址大型饱水木构件原址保护技术初探

本针对古遗址这一特殊建筑物,从环境地质的角度分析了古遗址及其大型木构件原址保护的主要环境地质病害,提出了“堵、截、排结合”的疏干排水方案、锚杆加固边坡和化学加固木构件的治理保护措施。并例举了湖北省铜绿山古铜矿遗址原址保护的环境地质病害的治理措施,和治理措施的预测的结果。     

Capillary seal capacity of faults under hydrodynamic conditions

Many fault bound traps are underfilled despite the top seal capacity being secure. The hydrocarbon sealing performance of faults themselves can be compromised either by mechanical or capillary process. Capillary process can be important either due to juxtaposition or to fine‐grained clay or cataclastic material within the fault zone itself. There is debate about how important each of these mechanisms is over geological timescales of hydrocarbon trapping. Recent work has provided insights into fine‐tuning capillary‐related fault seal calibration methodologies. Over the last 15 years, vigorous scientific debate with multiple published laboratory experiments and modelling studies has led some researchers and industry technologists to theorise that for water‐wet conventional hydrocarbon reservoirs, the relative water permeability in the reservoir (towards the top of the hydrocarbon column) may become very small, but in practice never reach zero. While not advocating for either side in this debate, the importance of accounting for hydrodynamic conditions regardless of the capillary sealing mechanism is demonstrated. Additionally, it is noted that nonzero relative water permeability has implications on how a seal's capillary threshold pressure for the nonwetting hydrocarbon phase is estimated from field data. In the particular case where there are pressure differences between unproduced hydrocarbon reservoirs on either side of a fault, then the hydrocarbon saturation must be discontinuous across the fault. For hydrocarbon leakage to occur across the entire thickness of the fault zone, the hydrocarbon pressure must exceed the threshold pressure on the side of the fault zone with the highest formation water hydraulic head. This approach to estimating across‐fault pressure difference will result in an improved calibration data set used for predrill estimation of capillary fault seal capacity.     

Hydrothermal fluid flow models of stratiform ore genesis in the McArthur Basin, Northern Territory, Australia

This paper explores the role of basin‐scale fluid migration in stratiform Pb–Zn ore formation in the southern McArthur Basin, Australia. Mathematical models are presented for coupled brine migration and heat transport in the basin. The models account for: (i) topographically driven flow (forced convection) during periods when parts of the McArthur Basin were subaerial and elevated above the central Batten Fault Zone; (ii) density‐driven flow (free convection) during periods when the basin was mostly submarine; and (iii) transient flows associated with fault rupture during periods of transpression. These hydrologic models help to compare and contrast a variety of hypotheses concerning deep fluid migration and the origin of base metal ores in the McArthur Basin. The numerical results exhibit a strong structural control on fluid flow caused by the north‐trending fault systems that characterize the Batten Fault Zone. As a result, fluids descend to depths of a few kilometers along the western side, migrate laterally to the east through the clastic and volcanic aquifers of the upper Tawallah and lowest McArthur Groups, and then ascend along the eastern side of the fault zone. This recharge–discharge pattern dominates all of the hydrogeologic models. The basin‐wide flow pattern suggests that Na–Ca–Cl brines acquired base metals in the deepest levels of the basin stratigraphy as the fluids migrated eastwards through the aquifer system. Upward flow was relatively rapid along the Emu Fault Zone, so much so that fluid temperatures likely approached 130°C in the muddy sediments near the sea floor due to upward flow and venting at the HYC (‘Here’s Your Chance'). Transient pulses of flow characterized periods of transpressional stress and subsequent faulting may have punctuated the basin history. Large‐scale free convection, however, characterized notably long periods of diagenesis and ore mineralization during the Proterozoic in the McArthur Basin.