I think it's being used for both. Could be split.

@@ -27,0 +28 @@
+<xref type="see" seq="1629060">炭酸飲料</xref>
@@ -28,0 +30 @@
+<gloss>carbonated water</gloss>

Meikyo daijr daijs koj

Do you have a source for that? The kokugos disagree. As do I actually, if somebody asks me 炭酸を飲む？ at dinner, I know they're asking if I want soda water, not coke or something similar.

GG5 does have this sentence: ►俺, 炭酸だめなんだよ.　I can't drink ┏carbonated beverages [fizzy drinks].
but I think this is just sense 1, a more literal translation would be "I don't like carbonic acid" and it's understood from context the speaker is talking about drinks.

Not exclusive to carbonated water. May refer to any carbonated drink in general.

@@ -29 +29 @@
-<gloss>carbonated water</gloss>
+<gloss>carbonated drink</gloss>

"Baking soda" and "baking powder" are different things.

@@ -36,1 +36,1 @@
-<gloss>baking powder</gloss>
+<gloss>baking soda</gloss>

http://www.kyoritsu-foods.co.jp/event/tansan/001.htm
http://www.kyousei-nara.jp/tansan-juso/index.htm
"タンサンとは別名　重曹ともいいます。全国的には重曹の方がとおりがいいかも知れ
ませんが関西ではタンサンの方がとおりがいいようです。"
http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q132
8949948
etc.

baking soda is sold under that name (at least where I live), 
in katakana (more than just one brand too, judging from google 
images)

@@ -16,0 +16,4 @@
+<r_ele>
+<reb>タンサン</reb>
+<re_nokanji/>
+</r_ele>
@@ -27,0 +31,8 @@
+<sense>
+<pos>&n;</pos>
+<xref type="see" seq="2171780">炭酸水素ナトリウム</xref>
+<misc>&uk;</misc>
+<misc>&abbr;</misc>
+<gloss>baking powder</gloss>
+<gloss>sodium carbonate</gloss>
+</sense>